Extension or Loft: The Cheapest Extra Bedroom

The denominator is the whole argument

A loft has three zones and only one of them is a room. Along the ridge there is full standing height. Out towards the eaves the ceiling comes down to meet the floor, and somewhere in between is a line past which the space is furniture-and-storage rather than habitable area. That line is not a matter of taste. Two different documents draw it, in two different jurisdictions, and both of them draw it well inside the roof.

Section R305.1 of the International Residential Code sets the minimum ceiling height for habitable space at seven feet, with an exception written for exactly this geometry: where the ceiling slopes, at least half of the required floor area has to be at the full seven feet, and no part below five feet counts towards the requirement at all. In England the line is drawn by the Technical housing standards — nationally described space standard, which asks for at least 2.3 m over at least 75 per cent of the gross internal area, and states that any area with headroom under 1.5 m is excluded from the gross internal area unless used solely for storage. That standard is a planning instrument applied where a council has adopted it rather than a building regulation — but as a definition of what a square metre of bedroom is, it is the one to divide by.

Both conventions agree on what matters here: there is a band of floor along the eaves you will pay to build, insulate, board and decorate, and that no measurement standard will let you call a bedroom. It is not wasted — it is where the wardrobes go, and eaves storage is worth having in a house short of cupboards. It simply does not belong in the denominator, and putting it there is the commonest way a loft ends up looking cheaper per metre than it is.

Marking it takes a tape and twenty minutes in the roof, and it has to be done before either builder is called back, because a builder measuring the same roof is under no obligation to use your threshold rather than the rectangle on the drawing.

An extension pays at the bottom and at the join

The peculiar thing about a rear extension is how little of the money is spent on the part you will see. Before any wall goes up there is a hole in the ground, and what that hole costs is decided by soil, by trees and by what the Victorians left buried across the middle of it. None of those three is visible on the drawing you were quoted against.

Foundation depth in shrinkable clay is set by the moisture the ground loses to nearby vegetation, and two documents are worth knowing by name before you take a price: NHBC Standards Chapter 4.2, Building near trees, which relates species, mature height and distance to a required depth, and BRE Digest 298, Low-rise building foundations: the influence of trees in clay soils, where the mechanism is explained rather than tabulated. BS 8004 covers foundations generally and BS 5837 covers what may be done to the tree itself. The consequence is stark: the same extension on the same street can want a trench a metre deep or three metres deep depending on a sycamore two doors down, and nobody can price that honestly until trial holes are dug.

Then there is the drain. Most rear extensions in an older terrace cross something, and since the transfer of private sewers under the adoption regulations made in 2011, that something in England and Wales is frequently a public sewer. Section H4 of Approved Document H covers building over or near a drain, and the sewerage undertaker's build-over agreement covers the rest — a fee, a timescale, and the standing possibility that the answer is a diversion. Where the new roof's rainwater goes to a soakaway, BRE Digest 365 is the method, starting from a percolation test on your own ground.

Near a boundary there is a statute rather than guidance. The Party Wall etc. Act 1996 catches building on the line of junction at section 1, work to a party structure at section 2, and — the one that surprises people — excavation near a neighbour's foundation at section 6, which bites within three metres to a lower depth than theirs and within six metres inside a forty-five degree line. Two months' notice for a party structure, one month for adjacent excavation, and a dissent means appointed surveyors you pay for. It is a lead time as much as a cost, and it has to start early enough not to hold the dig.

Above ground the extension carries two costs a new house does not: the structural opening into the back of the existing building, with a designed beam, propping and calculations for building control under Approved Document A, and making good — levelling the floor between old and new, plastering the junction, extending circuits and heating, redecorating every room the work crossed. Neither appears on a plan. Both appear on an invoice.

Which sets up the arithmetic the comparison turns on. Split the extension's cost into what scales with area — shell, roof, floor, finishes — and what does not: the opening, the party wall process, the drain agreement, the fees, the making good. The second group is a step you pay once whether the extension is three metres deep or five, so the last square metre is very nearly the cheapest thing in the project, and stopping short of the depth planning would have allowed is a free way to make the per-metre figure worse.

The roof decides before you do

A loft conversion is less a project you choose than one the roof permits or refuses, and the refusal comes on one of two grounds. The first is how the roof was built. A traditional cut roof — rafters, purlins and ceiling joists assembled in place — has a clear void and members that can be worked around. A trussed rafter roof, which is most houses built from the 1960s onward, is a set of engineered triangles whose webs run diagonally through the middle of the space you want, and no member of a truss can be cut without a replacement scheme designed to carry what it was carrying. That scheme is usually steel and a new structural floor before the conversion has properly begun. Not impossible; a different budget category, and the first thing to establish, because it moves the fixed step more than any other single fact about the house.

The second ground is height, and the arithmetic runs from both ends at once. Joist top to ridge underside gives the raw figure. Take away from the bottom, because the new floor is not the old ceiling and joists sized for a habitable room are deeper than what is up there. Then take away from the top, because the roof has to meet the fabric standard for a new thermal element in Approved Document L Volume 1, and that insulation lands between and under the rafters with plasterboard beneath it. What survives is the finished head height, and it is always less than the tape said first time.

The floor is not a detail. Existing ceiling joists hold up a ceiling and a few boxes; a bedroom is a different load case. IRC Table R301.5 puts uninhabitable attics without storage at 10 psf and with limited storage at 20 psf, against 30 psf for sleeping rooms and 40 psf for other habitable rooms, so the design load roughly trebles the day the space becomes a room. In the United Kingdom the equivalent is Category A of BS EN 1991-1-1 with its UK National Annex, read alongside Approved Document A. Either way the answer is new joists spanning wall to wall clear of the old ceiling, frequently on steels — engineered work with calculations attached, and firmly in the fixed step rather than the per-metre rate. Behind it come two smaller consequences: the roof stops being a ventilated cold void and becomes part of the heated envelope, which makes condensation a design question governed by BS 5250, and the new rooms need background and purge ventilation under Approved Document F.

Which brings back the dormer, and the dormer is the interesting one. It is a large fixed cost — a structural opening in the roof plane, cheeks, a roof over it, flashings and glazing — where rooflights cost a fraction and add no height at all. But a dormer widens the band of full head height across its whole width, which means it buys denominator. Price both versions, divide each by its own honest usable area, and the dormer scheme quite often wins on cost per square metre while losing badly on the headline. That comparison is the most useful thing a family can do with an evening, and almost nobody does it, because headline totals are what get discussed at the kitchen table.

1. Establish cut roof or trussed before anything else. A torch and ten seconds in the loft answers it, and it decides which budget you are in.
2. Measure joist top to ridge underside at the highest point, and write it down as the raw figure rather than the answer.
3. Subtract the new floor build-up from below — the joist depth an engineer will specify for a habitable load, not what is up there now — and the insulation and lining from above, at the thickness Approved Document L wants rather than the thickness that would fit.
4. Mark the loft floor where the finished ceiling meets your habitable threshold on each side, and measure between the marks. That is the room.
5. Repeat with a dormer assumed across part of the length, so you finish with two denominators for two totals.

The staircase is a room you are demolishing

Every loft conversion takes floor area off the storey below and hands it to a flight of stairs, and that transfer belongs in the comparison whether or not a quote mentions it. It is why a loft that looks like a gain of twenty-odd square metres is a net gain of seventeen — and seventeen is a different number to divide a total by.

How much it takes is arithmetic rather than opinion, because the stair's proportions are set by code. Approved Document K allows a private stair a maximum rise of 220 mm and a minimum going of 220 mm within a maximum pitch of forty-two degrees, and requires two metres of headroom — with a relaxation written specifically for this job, allowing 1.9 m at the centre of the stair width reducing to 1.8 m at the side where a sloping ceiling makes the full two impossible. Section R311.7 of the International Residential Code arrives at a similar shape by different numbers: 7¾ inch maximum riser, 10 inch minimum tread, 36 inch minimum width, 6 ft 8 in of headroom. Either way the going times the number of treads is the run, and the run has to lie flat on the floor of an existing bedroom or landing.

Work it through on your own floor-to-floor height and the number is uncomfortable. Divide the rise into risers at the maximum, take one off that count, multiply by the going for the run on plan, add the landing and multiply by the stair width: those are the square metres that have left the first floor. On many terraced houses the only place the flight fits is across the back bedroom — the room the new loft was meant to relieve. Doors move with it, and the room it lands in often ends up too narrow for the bed it was holding, so the scheme has produced a bedroom and demoted one in the same week. Plenty of houses take that happily. It is still a reason to count bedrooms of usable size before and after, on both schemes, rather than counting the new one and stopping.