Last reviewed: 2 April 2026 against MCS 020(a) Issue 1.0 Final. This page focuses on what the standard means operationally on domestic survey work rather than trying to reproduce the document.
For the broader service route behind this topic, see our Heat Pump Survey page.
Sound calculations matter because planning and permitted development questions often collapse into one simple customer concern: “Will this be allowed here?” The answer depends partly on location, partly on local planning context, and partly on whether the sound assessment is built from the right site information in the first place.
The practical rules that matter most
| Rule | What it means in practice |
|---|---|
| 37 dB limit | The assessment needs to stay within the permitted development noise limit at the relevant positions, not just feel “probably fine” on site. |
| Real unit, real location | The calculation only means something if it is tied to the actual model and the location the installer intends to use. |
| Boundary and neighbour context | Windows, boundaries, and neighbouring positions need to be clear enough that the office is not reconstructing the site later from memory. |
| Surrounding surfaces | Walls, hard standings, and the mounting choice all affect how comfortable you should feel with the proposed location. |
What the survey needs before the sound calculation is reliable
| Site detail | Why it matters |
|---|---|
| Exact outdoor-unit location | The calculation cannot be trusted if the location later shifts from what was originally assessed. |
| Boundary context | Distances and neighbouring positions are part of the practical risk picture. |
| Model selection | Quoted sound performance depends on the actual unit being proposed, not a generic placeholder. |
| Mounting and surrounding surfaces | Wall mounts, stands, and local surfaces can all affect how comfortable you feel with the chosen position. |
Worked example: why one site change can alter the result
| Scenario | Original assumption | Changed detail | Practical effect |
|---|---|---|---|
| Rear-garden ASHP on a semi-detached house | Unit proposed on open ground with a clear run to the boundary and no nearby hard reflecting surface noted | Survey photos later show a close masonry wall and paved area beside the chosen position | The office may need to revisit whether the assessed location still looks comfortable under MCS 020(a), rather than treating the first pass as final. |
| Model choice at quote stage | Office priced on a quieter shortlisted model | Installer later swaps to a different unit because of stock or lead time | The sound check may no longer sit on the same basis, so the planning conversation needs reopening before promises are made. |
Where problems usually begin
- The location is treated loosely at survey stage and becomes precise only after the quote is accepted.
- The office assumes the quietest model or the easiest location before either is fixed.
- Photos do not show enough boundary context to revisit the calculation later.
- The planning conversation is treated as separate from the survey rather than dependent on it.
Common reasons sites fail later than they should
- The proposed location moves after the visit but everyone still relies on the first assessment.
- A different outdoor unit is chosen after the quotation stage.
- Boundary photos are too weak to show the neighbour relationship clearly.
- Physical practicality wins on site day, but nobody circles back to check whether the sound basis still holds.
A practical site checklist
- Photograph the proposed location from more than one angle.
- Make the relationship to boundaries and neighbouring context obvious.
- Record the likely unit model or the realistic shortlist if the final choice is not fixed yet.
- Record any reason the “best acoustic location” may not be the best practical location.
- Do not promise a planning-free route before the sound and siting assumptions are actually checked.
Why this is still a survey quality issue
MCS 020(a) is a standards document. The on-site problem is usually simpler: did the survey leave the office with enough information to assess the location properly, or are people filling in the gaps afterwards from memory and optimism?
If you want to see how this fits into the wider planning question, the planning permission guide is the next page to read.