Siting an Air Source Heat Pump So It Stays Permitted Development

Twenty Minutes in the Garden, and the Homeowner Has Already Decided

She is pointing down the side return. It is out of sight from the road, it is where the bins live, the boiler flue is already through that wall, and it is the obvious place — which is exactly why it is the position most likely to cost her eight weeks. That passage is a metre and a half (5 ft) wide with a rendered wall on one side and the neighbour's rendered wall on the other, the neighbour's dining room window is nine metres (30 ft) down it, and the fence at the end is a gate. Every one of those facts changes the answer, and none of them are on the survey form.

The decision being made right now is not an aesthetic one. Three separate tests have to come out in favour of the same square metre of ground: the conditions that keep the installation inside permitted development, a noise calculation performed from a window that belongs to somebody who is not the customer, and the physical business of getting air into the coil, air away from the coil, and defrost water off the path. A position can pass two and fail the third, and the third is usually the one nobody checked.

So walk the garden in the order the tests get applied, not in the order the positions look attractive. Establish the machine first, because its size and its sound power are what everything downstream is measured against. Then run the noise arithmetic from every neighbouring opening facing the candidate. Then work out what the base costs and whether cable and pipe can reach it. Positions eliminated in that order stay eliminated; positions eliminated in the order they occur to the homeowner get re-argued at the second visit.

Permitted Development Is a Row of Gates, and Failing One Fails All of Them

Permitted development is a planning permission granted in advance by an Order rather than by the council, and it comes with conditions attached. The conditions are cumulative and they are not weighted: an installation that satisfies every condition but one is not mostly permitted, it is not permitted at all, and the route back is a householder planning application with an eight-week determination period and a neighbour consultation the homeowner did not want. That asymmetry is the whole reason this conversation happens in the garden and not after the equipment is ordered.

In England the right sits in Schedule 2, Part 14 of the Town and Country Planning (General Permitted Development) (England) Order 2015 — the renewable energy Part — in the class covering air source heat pumps on domestic premises. Wales, Scotland and Northern Ireland each have their own Order with its own numbering and its own limits, and the English class has been amended more than once. The amendments have moved exactly the numbers an installer is most tempted to quote from memory: the cased volume allowed for the outdoor unit, the distance the unit has to keep from the boundary of the curtilage, and how many pumps a property may have. Read the current consolidated text for the nation you are standing in rather than a figure from a wholesaler's brochure, a training course handout, or this page.

What has stayed put is the shape of the test, and that is the useful thing to carry around. Dimensional gates ask how big the box is and where it sits relative to boundaries, roofs and walls. Performance gates ask what the installation is for and how loud it is at somebody else's window. Status gates ask what the land and the building already are — designated, listed, scheduled, or subject to rights removed before you arrived. Each group is answered from a different document, and only one of them by looking at the heat pump.

Outside the UK the same twenty minutes happens against different objects. In the United States the setback is a zoning question decided by the municipality rather than by any model code, appliance access and clearance come from the locally adopted edition of the International Residential Code, and the noise limit is an ordinance written in dBA at a property line rather than at a window. What transfers is the habit: find out which document holds each answer before promising anything, because the one holding the noise answer is almost never the one holding the setback.

The Noise Test Is Arithmetic, and It Is Done From Someone Else's Window

The document is MCS 020, the MCS Planning Standards for permitted development installations of wind turbines and air source heat pumps located on or near domestic premises. It exists because the Order's conditions have long required a permitted development installation to meet the MCS Planning Standards or equivalent, which means the noise test is not a courtesy to the neighbour — it is one of the gates. A position that fails it is not a position with a complaint risk. It is a position that needs planning permission.

The test is a calculation, not a measurement, and it is performed before anything is installed. It fixes an assessment position one metre from the centre of the nearest habitable room window or door of a neighbouring property, and the calculated sound pressure level there must not exceed 42 dB(A). The inputs are the manufacturer's declared sound power level for that model, determined under BS EN 12102-1; the straight-line distance from the proposed unit position to the assessment position; a correction for the reflecting surfaces the unit sits against; and, where one genuinely exists, a barrier correction. The correction values come from the standard's own tables, and they are the part of this that should never be done from memory.

Two details decide more outcomes than the rest of the sum. The first is which opening counts. A habitable room is not a bathroom, a utility or a garage, and the definition also turns on whether a room is only a kitchen — which is the case that matters most on a terrace, because the nearest opening to a side return is so often the kitchen. Take that definition off the page of MCS 020 rather than from what a room looks like through a fence, because being wrong either way is expensive: discount an opening that does count and the assessment is worthless, count one that does not and a workable position has been given away. So the nearest window is not always the nearest window — and it is frequently upstairs, where the geometry is a diagonal rather than a ground-level line and where the fence you were relying on stops helping entirely. The second is that the distance is three-dimensional. A tape run along the ground to the base of a wall and then read off the plan is shorter than the real path to a first-floor opening, and short distances flatter the answer.

Run it for every candidate position and keep the worksheets. When a complaint arrives eighteen months later, the sheet naming the model, the position, the openings and the date is what shows the installation was assessed; its absence is what an authority reads as the assessment not having happened.

Passing MCS 020 is not the end of the noise question either, and it is worth saying so plainly to the customer. A calculated 42 dB(A) is a limit on level, and level is not the whole of what a neighbour hears. Statutory nuisance sits under Part III of the Environmental Protection Act 1990 and is assessed on its own terms; where an authority or a consultant looks harder, BS 4142:2014+A1:2019 is the method they reach for, and it applies character corrections for tonality and impulsivity that a single A-weighted figure does not carry. A fan tone sitting above the background at three in the morning can be a nuisance at a level that cleared the planning gate comfortably.

1. List every neighbouring property with a habitable room window or door facing the candidate position, at ground floor and at first floor, and mark each one on the site sketch.
2. Take the declared sound power level for the exact model and outdoor unit variant, at the operating condition it will actually run at rather than a quoted minimum.
3. Measure the straight-line distance from the proposed unit position to a point one metre outside the centre of each of those openings, in three dimensions.
4. Apply the correction for what the unit sits against — open ground, one wall behind, or an internal corner — from the standard's table rather than by judgement.
5. Apply a barrier correction only where a solid, continuous obstruction genuinely breaks the line of sight between the unit and that assessment position.
6. Compare the worst result against the limit, then record the worksheet with the model number, the dimensioned position, the date and a photograph of each opening.

Corners, Fences, and the Two Surfaces That Undo a Good Position

A unit standing on open ground is already radiating into a half space, because the ground beneath it reflects. Put a wall behind it and roughly half the energy that would have gone backwards comes forward instead — about three decibels. Stand it in an internal corner, with two walls and the ground, and the addition is around double that. Three decibels sounds academic until you remember the limit is absolute and not a target: a position calculating at 41 dB(A) in the open becomes a failure against the same neighbour, with the same machine, moved two metres into a corner. This is why the side return is the worst position commonly proposed and the one most commonly proposed. It is a hard-surfaced channel with two parallel reflecting walls, it usually runs directly beneath the neighbour's ground-floor and first-floor openings, and it usually restricts the airflow as well.

Fences behave differently from how homeowners expect. A close-boarded fence only attenuates where it is solid, continuous and actually interrupts the line of sight from the unit to the assessment position; a fence with 10 mm (3/8 in) gaps between boards is not an acoustic barrier, and a fence that is lower than the sight line does nothing at all. Worse, a screen built close to the unit stops being a barrier and starts being a reflector and an obstruction: check the manufacturer's minimum clearance to any surface on the intake and discharge faces before agreeing to an acoustic enclosure, because starving the coil raises the fan and compressor speed, and the extra noise arrives from the thing that was meant to reduce it. The honest advice on a marginal position is to move the unit, not to build a box around it.

Designated Land, Listed Buildings, and Rights Removed Before You Arrived

On designated land in England — conservation areas, National Parks, the Broads, areas of outstanding natural beauty and World Heritage Sites, the categories the Order gathers as article 2(3) land — the class is narrowed rather than withdrawn. The restriction bites principally on walls and roof slopes that front a highway, which is why the rear garden position frequently survives in a conservation area while the front elevation position never does. Misinformation runs in both directions here: homeowners are told the whole street needs permission when it does not, and installers assume nothing changes when the front wall clearly does.

Listing is a different mechanism and it is not affected by any of this. Listed building consent is a separate consent under the Planning (Listed Buildings and Conservation Areas) Act 1990, it is required for works affecting the special interest of the building, and permitted development rights say nothing about it — land within the curtilage of a listed building sits outside the class in any event. Scheduled monuments have their own consent regime under the Ancient Monuments and Archaeological Areas Act 1979. Then there are the rights that were taken away locally: an Article 4 direction can withdraw a class across a defined area, and many conservation areas have one, while on newer estates a condition attached to the original planning permission can remove permitted development rights for the whole plot. Both live on the authority's register, and both are checked by looking rather than by asking the customer. And if the chosen position needs a bracket cut into a party structure, the Party Wall etc. Act 1996 governs that quite separately from planning.

Everything the Position Decides Downstream

Whether the machine is a monobloc or a split changes what crosses the wall and therefore what the position costs. A monobloc keeps the refrigerant circuit factory-sealed in the outdoor unit and runs water to the house, which means freeze protection, external insulation and the hydraulic penalty of the run belong to you, and no handling of fluorinated gas is involved. A split runs refrigerant indoors, which brings the Fluorinated Greenhouse Gases Regulations 2015 and the certification that goes with them into the job, and it puts the manufacturer's maximum line length and maximum vertical separation directly into the siting decision — a position that clears every planning gate can still sit beyond the pipe run the model allows.

The pipe costs heat in both directions. Every metre of external or buried flow and return loses energy and pressure, and on a low flow temperature system that loss is a larger fraction of a smaller temperature difference than it would be on a boiler. Insulate to what the Building Regulations point at — in England, Approved Document L, Volume 1: Dwellings — and specify insulation that survives ultraviolet and vermin outdoors rather than the indoor product with a thickness that satisfies a table.

The electrical route is a siting input, not a consequence of one. The unit wants a dedicated circuit designed and protected to BS 7671, with voltage drop checked over the run to wherever the position ended up, an isolator within sight, notification under Approved Document P where the work falls within it, and the distribution network operator told about the new load. A position at the bottom of a long garden is a cable size decision as much as an acoustic one, and that cable is what usually makes the quiet position lose to the merely acceptable one.

And where the job replaces hot water storage, an unvented cylinder is a Building Regulations matter under Approved Document G with its own competent person requirements, and where it can go indoors often decides whether the outdoor unit can go on the far wall at all.

The Certificate That Ends the Argument

Where the position is genuinely marginal, a lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the instrument that settles it. It is a determination rather than a permission: the authority is deciding whether the thing you have described would be lawful, so the description has to be precise — the model, the cased dimensions, the position dimensioned from two fixed points, and the noise assessment attached. It is decided in the same eight weeks, it costs less than a full application, and what it produces is a document the homeowner can hand a conveyancer in a decade's time when a buyer's solicitor asks what that box on the side of the house is.

Use it where the boundary position is arguable, where the land is designated, where the neighbouring building is listed, or where the customer is nervous enough that a written answer is worth more than a fast start. And be clear about what a pre-application enquiry is not: an email from a duty planner is helpful and it is not a determination, it does not bind the authority, and quoting it back later carries no weight. Saying that out loud in the garden is the difference between managing an expectation and creating one.

What Goes on the Quote, and What Goes in the File

The quote should carry the position as a drawing, not as a sentence: a plan with the unit dimensioned from two fixed features, the boundary marked, the assessment openings marked, and photographs of the position and of each neighbouring window it was measured to. Attach the noise worksheet with the model number and the date on it. State which consents were checked and which were assumed — designation, Article 4, listing, and whether the boundary came from the title plan or from the fence — because an assumption written down is one the homeowner can correct, and an assumption in your head surfaces on installation day.

Then price the second position. Homeowners revisit this decision, usually after a conversation over a fence, and a quote that already carries the alternative and what it adds is a quote that survives that conversation instead of being reopened from the beginning. The second position also does something useful for you: it is the evidence that the first one was chosen rather than merely found.