MCS 020a noise rules explained
If you’ve read that air conditioning needs a “noise assessment” to qualify for permitted development and wondered what that actually involves, this is it: since 28 May 2026, MCS 020a is the only version of that assessment planning authorities in England accept. Older or informal noise estimates no longer count.
What MCS 020a is
MCS 020a is a standardised calculation, not a subjective judgement or a generic decibel rating printed on a spec sheet. It takes the specific outdoor unit’s sound output, its proposed position, and the distance and line of sight to the nearest habitable room window in a neighbouring property, and calculates the noise level that window would actually experience. If the result is under the permitted threshold, the noise requirement for permitted development is met at that position.
Why position matters more than the unit itself
The same outdoor unit can pass MCS 020a in one spot on a house and fail it in another. Distance to the nearest neighbouring window is the biggest factor, but orientation and what’s in between (a wall, a fence, open ground) also affect the result. That’s why a competent installer runs this calculation for the specific position being proposed, not for the unit in general: the unit’s rated noise output alone doesn’t tell you whether a given installation will pass.
In practice, this means the cheapest or most convenient mounting position isn’t always the one that clears MCS 020a. Sometimes moving a unit a short distance, or onto a different wall of the house, is what makes the difference between permitted development and a planning application.
What happens if it doesn’t pass
Failing MCS 020a at a given position doesn’t mean the installation can’t happen, it means that specific position doesn’t qualify for permitted development on noise grounds. The options from there are usually to try a different position, or to submit a planning application if the position genuinely can’t be moved.
Our approach
We run the MCS 020a calculation as a standard part of every quote where permitted development is in play, for the actual proposed position, not a generic estimate. If a position we’d otherwise use doesn’t clear it, we’ll say so and suggest an alternative before you book, rather than leaving it for you to discover once the unit’s already been ordered. For the wider picture on when this calculation is even relevant, see our air conditioning planning permission guide.