Heat pump permitted development in 2026: the noise assessment that decides the job

Permitted development is the quiet part of a heat pump job. Nobody talks about it in the sales conversation, it rarely comes up on the survey, and most of the time it just works. Then one install in twenty has a neighbour's window in the wrong place, the noise assessment comes in too high, and a three-week job turns into an eight-week wait for a householder planning application. The margin you priced in is gone, and the customer is asking why the timeline moved.

The rules changed in the installer's favour over the last year - but they also moved the whole decision onto one calculation you have to get right at survey stage. Here is what actually applies now.

What changed, and when

Two dates matter, and they are a year apart.

29 May 2025 - the Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2025 came into force and relaxed the physical limits on domestic air source heat pumps:

• The one-metre-from-the-boundary rule is gone. You can now site a unit within a metre of the property boundary. The old rule was a blunt stand-in for noise control; it has been replaced by an actual noise test (more on that below). Note this is specifically the boundary rule: a separate one-metre requirement still applies on flat roofs, where all parts of the unit must sit at least a metre from the external edge of the roof.
• The outdoor unit size limit rose from 0.6 m³ to 1.5 m³ on a house (it stays at 0.6 m³ for a block of flats). That takes almost every domestic monobloc out of the "too big for PD" bracket.
• Detached houses can now have two units under permitted development, not one - useful for larger properties or a heat-plus-cooling setup. On a non-detached house or a block of flats, it is still the first installation only.
• Air-to-air units that also provide cooling are supported.

28 May 2026 - the noise side of the right tightened up. MCS 020(a), the sound calculation standard, is now the sole permitted scheme for demonstrating that an installation meets the permitted development noise condition. There is no longer an alternative route: if the job is going in under PD, the assessment is done to MCS 020(a), full stop. (Using the standard has been mandatory for PD installs since 20 September 2025; the 28 May 2026 change removed the last alternatives.)

The headline reads like deregulation, and for the physical siting it is. But permitted development for an air source heat pump has always been conditional, and the binding condition is now the noise assessment. Get that wrong and none of the relaxations help you - the installation simply isn't permitted development, and it needs planning permission before it goes in.

The number the whole thing turns on: 42 dB(A)

Permitted development requires that the sound from the heat pump does not exceed 42 dB(A) at the assessment position - the nearest habitable room at the neighbouring property most likely to be affected. MCS 020(a) is the method you use to prove, before you install, that you will land under that figure.

This is where jobs quietly fail, because the calculation is less forgiving than it looks:

• It uses the unit's sound power level, not its sound pressure level. These are different quantities, and picking the wrong one off the datasheet throws the whole result out. Sound power is the figure you feed in.
• Reflective surfaces within one metre of the unit push the number up. Under MCS 020(a), a reflective surface is any hard surface within a metre of the heat pump - including the ground, but not vegetation. A unit tucked into a corner between two walls behaves very differently from one on an open patch of wall, and the calculation reflects that.
• The distance and line to the neighbour's window drive the result far more than most people expect. The relaxed boundary rule means you can site closer to the boundary - but if that boundary is a few metres from a neighbour's bedroom window, the assessment is what tells you whether you actually should.
• Two units go into one calculation. MCS 020(a) now includes a method for assessing up to two air source heat pumps together, which is what makes the "two units on a detached house" allowance usable in practice rather than in theory.

None of this is difficult once it is part of the survey. The failure mode is treating it as a box-tick after the fact, discovering at design stage that the chosen location won't pass, and having to move the unit, add an acoustic barrier, or fall back to a planning application.

Why this is an operations problem, not a paperwork problem

The cost of getting permitted development wrong is rarely the assessment itself. It is everything downstream:

• A location chosen on site that later fails the calculation means a re-survey, a redesign, and sometimes an awkward second conversation with the homeowner about where the unit is going.
• A job that unexpectedly needs planning permission adds around eight weeks and a fee, and it lands after you have already given the customer a date.
• If the noise assessment isn't recorded and kept, you have nothing to point to if a neighbour complains after commissioning - and no evidence the installation was permitted development in the first place.

The installers who handle this cleanly do the same thing every time: they capture the noise-relevant facts at survey - unit model and its sound power figure, the proposed location, what is within a metre of it, and the distance and sightline to the nearest neighbouring window - and they run the MCS 020(a) check before the design is signed off, not after. The location decision is made once, with the number in hand.

That is a data-capture habit as much as a compliance one. In Payaca you can build the site-survey inputs into a custom fieldset on the project, attach the completed MCS 020 assessment and the MCS certificate to the same record, and keep them against the job rather than in someone's inbox - so when a customer or a neighbour asks two years later, the permitted development evidence is where the job is. The calculation is still yours to run to the standard; the point is that the inputs and the record live in one place instead of being reconstructed from memory.

Key takeaways

• Since 29 May 2025, domestic air source heat pumps in England have had the 1m boundary rule removed, a larger 1.5 m³ unit size limit on houses, and up to two units on detached houses.
• Permitted development is conditional on noise: the installation must come in at or under 42 dB(A) at the nearest neighbouring habitable room.
• From 28 May 2026, MCS 020(a) is the only permitted scheme for that noise assessment - there is no alternative route.
• The calculation uses sound power, not sound pressure, penalises reflective surfaces within one metre, and can now assess two units together.
• Do the assessment at survey, not after design. A location that fails the calculation is a re-survey at best and a planning application at worst.

Keeping the survey inputs, the noise assessment and the certificate on one project record is the difference between permitted development being a non-event and it being the thing that blew your timeline. If your team is still reconstructing that paperwork job by job, book a demo and we'll show you how installers keep it in one place.

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Sources: The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2025; Planning Portal - Air source heat pump permitted development; MCS 020(a) Air Source Heat Pump Sound Calculation. Figures current at time of writing (July 2026); confirm against the primary sources for each job.