Three weeks of a cistern refilling, and a floor that was already soft
It refills every eleven minutes, and it has been doing it since some point in March. You can hear it from the landing at two in the morning, which is how it stopped being background noise and became a job. The fix is a flapper or a fill-valve seat, it costs less than a takeaway, and it takes about a quarter of an hour with the water off. That is not why you are reading this.
You are reading this because when the bath panel came off to get at the isolating valve, the chipboard along the front edge of the bath had gone the colour and consistency of digestive biscuit, and the silicone at the wall junction turned out to have parted from the tile some years ago rather than some months ago. A running cistern is a component failure. What was found behind the panel is a room, and the two have nothing to do with each other except that one led you to the other.
So there are two arithmetics on the table now and they are not the same size. One prices a room — small wet room, open shower, gully in the floor, everything back to the joists. The other prices what the cistern has been quietly costing since March, and it decides a date rather than a budget. Confusing them is the usual thing that happens here, generally in the direction of believing the leak somehow contributes to the refit. It does not, and the arithmetic that shows why is worth doing early, because it is what stops you deferring a flapper for six months while you save up for tiles.
The room is priced through two areas and a count, and floor area is not one of them
Cost per square metre is the working convention almost everywhere else in construction. It is how an extension gets an early figure, how a self-build is benchmarked, and how the elemental cost plans in RICS NRM 1 are structured. Applied to a bathroom it produces numbers that look like typing errors, and the reason is not that bathrooms are mysteriously expensive. It is that the denominator has almost nothing to do with the cost.
Ask which lines move when the floor gets bigger. The fixture count does not: four fixtures in a 3.9 m² room are four fixtures in a 10 m² room. The extract fan is one fan, its duct run is set by the distance to an outside wall rather than by the area it serves, and the electrical work is one circuit, one isolator and one set of luminaires wherever it goes. The shower enclosure is one shower. Of the eight or nine lines in a bathroom estimate, exactly two scale with area, and one of those — the tanking — scales with the wet zone rather than with the room.
The line that does follow area follows it badly, because tiling follows surface and surface is dominated by walls. Wall area is perimeter times height, and perimeter grows with the square root of area for a room of fixed proportions, so the smaller the room the more wall each square metre of floor drags behind it. The three rectangles below are tiled full height to 2.1 m with no deduction for openings, so the geometry stays visible. The largest has two and a half times the floor of the smallest and buys less than double the tiling for it — and precisely the same four fixtures.
So the estimate is elemental: a count multiplied by a rate per fixture point, two separate areas multiplied by two separate rates, two lump allowances that do not scale at all, and a contingency over the sum. The rest of this page is about getting the count and the two areas right. The rates are yours, from your own quotes in your own market, and this site does not publish them.
| Room, internal | Floor | Wall tiled to 2.1 m | Wall m² per m² of floor | Total tiled surface per m² of floor |
|---|---|---|---|---|
| 1.70 × 2.30 m | 3.91 m² | 16.80 m² | 4.30 | 5.30 |
| 2.40 × 2.60 m | 6.24 m² | 21.00 m² | 3.37 | 4.37 |
| 3.00 × 3.40 m | 10.20 m² | 26.88 m² | 2.64 | 3.64 |
What a fixture point actually contains
A fixture point is not an object in a catalogue. It is a hot supply or a cold one or both, terminated and isolated; a waste with a trap at the right seal depth, connected into a branch running at a fall; a fixing into whatever is behind the plasterboard; and the labour of a second visit, because every sanitary fixture in a refit is connected twice — loose for first fix and testing, properly once the tiling is grouted. That double handling is why the point is the unit and the item is not.
Count what a plumber connects rather than what a showroom photographs. A bath with a mixer on it is one point; the same bath with a separate wall valve and riser is two. A heated towel rail is not a sanitary fixture but it is a pair of connections and a decision about whether it sits on the heating circuit or on an element, so it belongs in the count. A dead pipe inside the wall is a point too, because someone has to find it, prove it is dead and cap it.
The count has one hard boundary and it is worth stating before the estimate is built, because crossing it invalidates everything downstream. Replacing a fixture where it already stands is fit-out work. Moving one to a materially different position is a drainage alteration: the branch has to be re-run at a fall it may not have the depth for, which usually means lifting the floor, and a WC is the worst case because its trap arm is the shallowest thing in the room. If any fixture is moving more than a short distance, price the drainage as its own exercise first and treat the room estimate as conditional on the answer.
Tanked area and tiled area are different numbers, and the codes set the smaller one
Tile is not a waterproof layer and grout is emphatically not one — cementitious grout passes water under a shower head as a matter of routine, being a joint filler with a service life rather than a barrier, and backer board is water-resistant rather than waterproof. What keeps water out of the structure is a separate system underneath, either a liquid applied in coats over a primer with reinforcing tape at every junction, or a bonded sheet lapped and sealed. It has its own material cost, its own labour, its own cure time and its own performance standard — ANSI A118.10 in North American specifications — and it is a separate line in any honest estimate.
How much of it you need is one of the few things in a bathroom that a document decides rather than a preference. The International Residential Code, at Section R307, requires shower compartment walls and the walls above bathtubs with shower heads to be finished with a nonabsorbent surface to at least 1,829 mm above the floor. AS 3740 sets the waterproofing extents for domestic wet areas in Australia in more detail — the whole floor of a shower area, the wall-to-floor junction taken up a short height elsewhere in the room, and shower walls taken far higher. BS 5385-4 is the UK code of practice for tiling in wet and other special conditions. Heights and extents differ, so use the one that applies where the work is; the shape of the answer does not change. The wet zone is a defined region, smaller than the tiled area and larger than the shower tray.
Take the small room from the table — 1.70 by 2.30 m internally, 3.91 m² of floor, 8.00 m of perimeter — and measure both areas properly. Tiled: walls to 2.1 m is 16.80 m², less a 0.80 by 2.00 m door opening at 1.60 m², plus the floor, giving 19.11 m². Tanked to the AS 3740 minimum with a linear gully against the 1.70 m end wall: the whole floor at 3.91 m², plus the shower-zone walls — the 1.70 m end wall and 0.90 m of return on each side, so 3.50 m at 1.80 m high, or 6.30 m² — plus the remaining 4.50 m of perimeter taken up 150 mm at the junction, 0.68 m². Total 10.89 m². Three numbers for one room: 3.9, 10.9 and 19.1 m², and only the middle one is what the membrane is priced against.
In a genuine wet room on a timber floor, plenty of specifiers tank the whole envelope rather than the code minimum, and that is a defensible choice rather than gold-plating — the floor is the thing that has already failed in this house. But it is a choice with a price, so price it both ways and make it deliberately, which is only possible if the estimate carries tanked area and tiled area as two independent inputs. It is also how you sanity-check a quote you did not write: if a breakdown rolls tiling and waterproofing into one square-metre rate against one area, ask which area it was multiplied by. Either the membrane is being spread over surfaces that do not need it, or — far more often — the tanking has quietly become a percentage of a rate rather than a system with a data sheet.
Assembling the room from the parts that do not scale
With the count and the two areas measured, the estimate is arithmetic: four fixture points at your fitted rate, 19.1 m² of tiling, 10.9 m² of tanking at the membrane system's rate, one allowance for the fan with its duct and supply, one for lighting and decorating, and a contingency over all of it. The result worth reading is not the total but the cost per fixture that falls out of the bottom — the only figure on the page that compares across rooms of different sizes, and therefore the one to carry to a second quote.
Leave every rate at zero until you have a real one. A half-remembered rate produces a total that looks authoritative and is not, and the failure mode is specific: it gets quoted to a partner, becomes the budget, and then survives contact with the first real quote by being argued with rather than replaced. Two things also stay outside the arithmetic on purpose — repositioning the WC, which is a drainage alteration rather than a fit-out line, and structural repair to a floor found inadequate once it is lifted, which is the subject of the contingency section below.
Put the fixture count in as points a plumber connects, not as items in a catalogue, and keep tiled area and tanked area as the two different numbers you measured — 19.1 and 10.9 m² for the worked room here. Then read the cost-per-fixture line in the breakdown rather than the total: it is what survives a comparison against a bathroom that is not the same size as yours.
Bath, shower, basin, WC, bidet — each counts as one.
Item, pipework and fitting labour per fixture.
Walls plus floor.
Tile, adhesive, grout and labour.
Wet zone requiring a membrane.
Membrane or liquid system, applied.
Fan, ducting and its electrical supply.
Lighting, shaver point, heated rail, paint.
Higher where the existing floor is unknown.
Total bathroom cost
Needs your rates
This page does not assume a price. Enter yours and the answer appears here.
They open the calculator with your figures already in it
Bathroom Remodel Cost Calculator — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Excludes moving the soil stack or the WC to a materially different position, which changes the drainage design rather than the fit-out.
- Excludes structural repair to a floor found to be inadequate once lifted — a common discovery and the main reason the contingency here is set higher than for other rooms.
Why the labour per square metre looks wrong until you draw the programme
Every trade rate you have from the rest of the house will under-price this room, and the reason is not that bathroom fitters charge more. It is that three separate things about a small wet room make labour dense in a way that a rate per square metre cannot express.
The first is cut density. In a room 1.70 m wide with four fixtures, a linear gully, a fall in the floor and a door lining, very nearly every tile meets something. Cuts are the unit of tiling labour, not area, and cut count is set by perimeter and obstacles — both proportionally largest in the smallest room. The fall compounds it: small-format tile is what lets a floor fall to a gully without lipping, and small format means more units, more grout line and more setting time per square metre than the field tile anywhere else in the house.
The second is that a small bathroom is a queue rather than a workforce. One person works comfortably in a 1.70 m width; two get in each other's way, and the tiler cannot start while the first-fix pipework is under test. Adding labour does not compress the programme, so every rate that assumes trades overlap is optimistic here. The third is cure and set time — dead calendar with no labour in it at all. The primer wants its stated open time, the membrane wants its coats and the interval between them, the flood test wants its dwell, the adhesive wants its set before grouting and the grout wants its cure before silicone. None of it is chargeable hours and all of it is days the room does not exist, which is also what decides whether the household needs somewhere else to wash. The sequence below is the ordinary one for a single-bathroom house, and its waits are why a room this size takes a fortnight rather than a long weekend.
- Strip out, cap off, expose the deck. The first real information about the job arrives here, not in any quote.
- Structural repair and deck preparation if the exposed joists and boards need it. Everything after this assumes a sound floor.
- First fix: supplies re-run, waste branches set to their falls, gully body set from the bottom up, fan duct taken to its outside terminal.
- Pressure and drainage tests while everything is still visible, with the electrical first fix in the same window.
- Form the falls, prime, then tank in coats with reinforcing tape at every junction and penetration — at the stated interval between coats, not an assumed one.
- Flood test and leave it standing. Last moment a failure is cheap.
- Tile, let the adhesive set, grout, let the grout cure.
- Second fix fixtures, fan, lighting and towel rail; silicone the movement joints last.
What the cistern has been costing since March
Now price the thing that started this, because it is the one number here that a delay changes. A cistern refilling on a cycle is leaking past the flush valve seal continuously between refills, and the rate is measurable rather than guessable: shut the fill valve, mark the water line and time the drop, or catch the flow at the pan with a container and a watch. The commonly cited figure for a running toilet is around 0.125 litres a minute, which is 65,700 litres a year — about what Approved Document G's water-efficiency requirement allows a person and a half.
Two things about the rate you multiply that by. Do not divide the bill by the volume: the fixed service charge is payable whether the house uses anything or not, and on a tiered tariff the water a leak wastes is the last water, priced in the highest block the household reaches. Take the marginal rate off your own bill instead — the water-bill guide linked at the foot of this page works through how, including the unit traps between a register in cubic feet and a tariff in thousands of gallons. Then add the sewer charge. Where wastewater is billed volumetrically against metered water this leak is billed twice over, because unlike a leak on a buried supply pipe, every litre of it goes through the pan and into the drain.
Then read the answer for what it is. Set the annual figure next to the room total from the section above and the ratio is stark: the leak is a rounding error against the refit, and anyone presenting it as an offset is selling something. What the number genuinely prices is the twelve months you might spend saving up — and whether to put a flapper in this weekend and do the room next spring. It nearly always is worth it, because the seal is a consumable part, replacing it needs no trade and no tiling, and it comes out again with the cistern when the refit happens. The arithmetic also misses the second reason: a cistern that runs continuously keeps the pan seal and the overflow route permanently wet, and in a floor already found soft along the bath edge that is not a neutral condition to leave for a year.
Measure your own leak rate rather than accepting the default — a slow weep and a valve that never quite seats differ by more than a factor of ten. For the rate, use the marginal water charge from your own bill plus the volumetric sewer charge added together, because this leak reaches the drain and is billed on both. The result is the cost of waiting, not a discount on the refit.
How fast the leak flows, in the unit shown.
Your local water utility's volumetric rate, in the unit shown.
Water wasted per year
17,400 gal
Figures that depend on a rate wait for yours — this page does not assume one.
They open the calculator with your figures already in it
Water Leak Cost Calculator: 17,356 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- A leak on the hot side costs the water and the energy that heated it. Raising the same volume 40-50 °C (70-90 °F) at typical fuel prices runs several times the water charge itself, so a dripping mixer or a hot supply weeping past a valve is mostly a fuel bill wearing a water bill's clothes. This figure sees only the volume.
- The result runs the entered rate for all 525,600 minutes in the year. A flapper that only weeps for a while after each flush, an irrigation line that is only live in season, or a leak that has been steadily worsening each has a duty cycle, and cost scales straight in proportion to it — time the minutes it actually runs in a day and pro-rate, rather than accepting the continuous-flow assumption baked in here.
The rebate is attached to the fixture, not to the room
Water-efficiency incentives are almost always local. There is no general federal water-efficiency tax credit in the United States the way there is for energy work — the schemes paying for efficient toilets and showerheads are run by utilities, municipalities and states, and the EPA's WaterSense programme maintains a finder for them rather than funding them. In the UK efficiency is regulatory rather than incentivised, with the Water Supply (Water Fittings) Regulations capping WC flush volume and Approved Document G setting a consumption requirement per person per day. In Australia, WELS is a labelling and minimum-standards scheme with state and utility rebates layered on top. Establish which shape applies before assuming any number at all.
Whatever the scheme, the structural point is the same and it is where the arithmetic usually goes wrong: the rebate attaches to a qualifying fixture, not to the project. A utility paying for a WaterSense-labelled toilet is paying against the pan, the cistern and its label — not against your tanking, your tiling, your extract fan or the two days of labour that put the thing in. The gross you feed into a rebate calculation is a qualifying subtotal, usually a small fraction of the room, and programmes stack conditions on top: the model must appear on a published list, the old fixture may have to be surrendered or photographed, and there is frequently a cap per fixture and a limit per household.
Know which of the two mechanics you have, because they behave differently. A flat rebate is money paid or credited against a purchase, and for water fixtures that is nearly always the form it takes. A percentage credit is a tax instrument: it reduces what you owe when you file, so you need enough liability to use it, and it arrives at a different time of year from the invoice. In a bathroom the percentage line is usually zero unless the refit includes a piece of qualifying energy equipment such as a heat-pump water heater — in which case that appliance's own cost is the gross it applies to, and not the room.
There is one order-of-operations trap worth spelling out, because it is silent. The calculator below works out the percentage against the gross cost and then subtracts the flat rebate: on 4,000 gross with a 30 per cent credit and a 500 flat rebate, that is 4,000 less 1,200 less 500, or 2,300. Some programmes work the other way round and require the utility rebate to be deducted before the credit is computed, which gives 3,500 times 0.7, or 2,450. The two differ by the percentage of the flat rebate — 150 here — and neither number announces itself as the wrong one. If your programme deducts first, model it by entering the reduced gross of 3,500 with the flat rebate left at zero.
Last, do not count the same saving twice. Replacing a 13-litre cistern with a modern one is a genuine ongoing reduction, but it is largely the same water the leak section has already priced. Price the leak, or price the efficiency gain against a properly working old fixture — not both.
Enter the qualifying subtotal as the gross, not the room total — the fixtures a programme actually names, at their purchase price. Leave the percentage at zero unless a genuine tax credit applies to a specific appliance, and check whether your programme wants the flat rebate deducted before the percentage is worked out rather than after.
The full price before any incentives are applied.
Any percentage-based incentive, like a federal tax credit.
Any additional flat-dollar rebate, like a utility or state program.
Net cost after rebates
$10,000
Tax credits typically reduce what you owe in taxes rather than being paid upfront like a rebate — confirm the specific mechanism and eligibility requirements for your program before counting on the savings.
- Total rebates/credits
- $0
They open the calculator with your figures already in it
Rebate-Adjusted Cost Calculator: 10,000 $ (net cost) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- The percentage is taken on the full gross figure before the flat amount is removed, so where a program requires a utility rebate to be deducted first and the credit worked out on what remains, the credit shown here is larger than the one you will actually receive.
- Every dollar you enter as gross cost is treated as qualifying, so permits, structural repair, disposal or other line items a program excludes from its eligible base are not separated out — enter only the eligible portion when the incentive covers part of the invoice.
- Nothing in the arithmetic caps either incentive: the percentage and the flat amount come off in full whatever you type, so a program's maximum dollar value has to be checked and imposed by hand before the net figure means anything.
- The subtraction is never floored at zero, so a flat rebate bigger than the balance left after the percentage produces a negative net cost rather than a fully covered project.
- Only one percentage slot and one flat slot exist, and the breakdown adds them into a single rebate total, so stacking several programs means summing them yourself — which matches reality only if each one is genuinely worked out on the same full base rather than on what the previous incentive left behind.
Contingency in a room whose floor nobody has looked at
Standing advice for renovation contingency lands around ten to fifteen per cent. In a bathroom over a timber floor in an older house that is too low, and this bathroom has already told you why: the deck along the bath edge has failed. Twenty-five is the defensible number, and it is not pessimism — the estimate above was priced against a floor nobody has seen.
What gets found is a short and repetitive list. Joists notched to clear the old waste run, usually deeper and nearer a bearing than IRC Section R502.8 or the notching guidance behind Approved Document A permits, which turns a fit-out into a repair with an engineer's opinion attached. A deck too flexible for what is going on top of it — the tile trade works to the TCNA Handbook's deflection criteria, L/360 for ceramic and a considerably stiffer L/720 for natural stone, and a floor that was fine under vinyl is not automatically fine under stone and a tanking system. A soil branch in lead or cast iron with a joint that will not take a modern connection. Bearers rotted where a previous tray leaked and was made good with more silicone.
None of that is in the estimate and none of it can be, which is the honest position rather than a hedge — the calculator says as much in its own limitations. Set the contingency at twenty-five and treat the first day of the strip-out as the moment the estimate either firms up or gets rewritten. If the deck comes up sound you have a contingency you get to keep, which is a considerably better problem than the alternative.
Three things that are cheap now and impossible afterwards
The tanking is the obvious one, worth restating in cost terms rather than technical ones: it sits under the tile, it cannot be inspected once the tile is on, it cannot be repaired without removing everything above it, and it is a single-figure percentage of the room. A shower built without it damages the floor beneath invisibly, and the discovery looks exactly like the bath panel that started this.
The other two are duller. The extract fan has to discharge outside rather than into a roof space — terminating in a loft relocates moisture instead of removing it — and the rates it must move come from ASHRAE 62.2 for local exhaust or Approved Document F, depending on where you are. And the electrical work in a room containing a bath or shower is zoned: Section 701 of BS 7671 in the UK, and in the US a dedicated 20 A branch circuit with GFCI protection for receptacles under the NEC. Both are first-fix decisions taken behind plasterboard on a day the room is open, and both cost a fraction of what they cost as a retrofit through a finished tiled wall.
What to have on paper before you ask anyone for a price
Six things, all obtainable with a tape, a bucket, a watch and half an hour in the room you already own. None of them depends on anybody's rates, and each one changes a line that a quote will otherwise fill in for you.
- Internal length and width, and the perimeter walked through every return and reveal — The perimeter is what wall area is built from, and in a room this size wall area is four times the floor. Measure around the actual shape rather than assuming a rectangle: a boxed stack or a chimney breast adds perimeter and adds tiling.
- Tiled area: perimeter times the tiling height, less openings, plus the floor — Decide the height before you measure it. Full height to 2.1 m and half height to 1.2 m are different jobs, and the difference is not proportional because the top course still needs cutting either way.
- Tanked area: the wet zone measured to the standard that applies where you are — Floor, shower-zone walls to the height the code sets, junctions taken up around the rest of the room. AS 3740 sets waterproofing extents directly, while IRC Section R307 sets a nonabsorbent finish height rather than a membrane extent — so use the rule you are building under, and note the whole-envelope alternative as a separate figure so it can be priced as a choice.
- Fixture points, counted as connections rather than as items — Bath, shower valve, basin, WC, towel rail, and any capped pipe that has to be found and made safe. Mark which of them are staying exactly where they are — the ones that are not turn the job into a drainage alteration.
- The cistern's leak rate, timed with the fill valve shut — Mark the water line, wait a measured interval, measure the drop and convert against the cistern's plan area — or catch it at the pan. Guessing this is the difference between a trivial cost and a substantial one, and the two look identical from the landing.
- Marginal water rate plus the volumetric sewer rate, both lifted off your own bill — Not the bill divided by the volume, which folds in the fixed charge. This leak reaches the drain, so both charges apply to every litre of it.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.