Part O and noise: the openable window conflict

Two regimes ask for opposite things at the same window. Planning, through the noise assessment, wants bedroom windows shut at night so internal levels meet the guideline values. Approved Document O wants windows open so heat can escape during the night, and it explicitly refuses to let you rely on openable windows where noise means they will be closed. On a site next to a main road or a late-licensed venue, that is a direct conflict, and it has to be resolved in the design rather than in the paperwork.

The resolution is usually mechanical: ventilation and, on the hardest sites, cooling. Getting there is straightforward once the two assessments are run against each other rather than in separate silos by separate consultants who never speak.

Why the two requirements pull in opposite directions

The noise case is familiar. Internal guideline values are commonly taken from BS 8233, with 35 dB LAeq during the day in living rooms, 30 dB LAeq at night in bedrooms and a maximum level criterion for individual events. On an exposed facade those are achievable with glazing and an alternative ventilation path, and unachievable with the window open.

Part O works the other way. It asks whether the dwelling will overheat, and its simplified method depends on free area of opening for cross ventilation and night purge. Take the openable window away and the simplified route usually fails.

So the same facade is being asked to be sealed at night for one regime and open at night for the other. Anyone who has stood in a new flat on a July night next to a bus route knows how this ends in practice, with the resident choosing sleep over temperature or the other way round.

How Part O treats noise at night

Approved Document O deals with this head on. Where noise would mean windows are shut during the night period, the overheating strategy cannot count on them being open.

The document sets the test at internal levels with windows open during the night, using an equivalent continuous level over the eight-hour night period together with a maximum level criterion that must not be exceeded more than a limited number of times a night. Where either is breached, windows are treated as closed for the purposes of the overheating assessment.

Take the exact figures from the current edition of Approved Document O rather than from a summary, because they are the numbers the building control body will apply and they have changed once already.

The practical consequence is that your acoustician's facade calculation decides which overheating route is available to you. That is a sequencing point as much as a technical one.

The free running versus sealed building decision

This is the fork in the road, and it should be taken at concept stage.

A free running building relies on openable windows, cross ventilation and night purge. It is simpler, less expensive to build, has no plant to maintain and performs well on a quiet site. It needs a facade where windows can actually be opened.

A sealed or partially sealed building uses mechanical ventilation with heat recovery, acoustic attenuation on the supply and extract paths, and often some form of cooling. It costs more in capital and in running energy, it needs plant space, maintenance access and a management regime, and it works where the free running option does not.

Deciding late is what causes redesigns. A scheme designed as free running that discovers at technical design stage it needs mechanical ventilation and cooling has to find riser space, plant space, an external condenser location and the acoustic treatment for that plant, none of which were in the planning drawings.

Mechanical ventilation and cooling as an answer

Mechanical ventilation with heat recovery solves the ventilation half. It supplies fresh air through an attenuated path so the window can stay shut, and a summer bypass stops it recovering heat you do not want. It does not, on its own, remove heat quickly enough to solve overheating on a hot night in a highly glazed flat.

Additional measures in the order they should be considered: reduce the heat gain first through glazing ratio, glass specification and orientation, then shade with external shutters, brise soleil, balconies and deep reveals, then increase the ventilation rate through attenuated louvres or acoustically treated ventilation paths, and only then add active cooling.

That order matters in London, where the cooling hierarchy in the London Plan requires it, and it matters everywhere for the energy consequence. It should be shown explicitly in the Energy Statement alongside the overheating work.

External plant brings its own noise problem. A condenser on a roof or a balcony is an industrial or commercial sound source assessed against the background level, and it can create the very impact your facade design was solving.

What planning and building control each control

Part O compliance is determined by the building control body, not by the planning authority. Planning controls what the building looks like, where the plant sits, what noise it makes, and the mitigation secured by condition.

The overlap is the problem. Conditions requiring internal noise levels to be met with windows closed, plus an approved ventilation strategy, are standard. If those conditions are discharged with a strategy that is inconsistent with the overheating route being pursued at building control, you end up varying one of them.

Anything that shows on the outside has to be in the planning application: external shading, louvres and vents, condenser positions and their acoustic screening, and any change in glazing proportion. Adding them later usually needs an application under section 96A or section 73 of the Town and Country Planning Act 1990, depending on whether the change is material.

Presenting a consistent strategy across both regimes

Run the two assessments together and present them as one narrative. The Noise Impact Assessment establishes facade levels and the internal levels achievable with windows open and closed. The Overheating Risk Assessment states which Part O route is used and what it assumes about window operation. The ventilation strategy states the path, the attenuation and the plant.

Then add a short compatibility statement: for each unit type, the assumed window condition at night, the ventilation provision, the overheating route and the noise mitigation, on one page. Officers and their environmental health colleagues want to see that the two teams have read each other's work. Glazing and ventilation specifications end up in conditions, and our note on noise insulation and glazing specifications in conditions covers how to draft them so they can be discharged.

Design moves that reduce the conflict

The best answers are architectural rather than mechanical.

Put habitable rooms, particularly bedrooms, on the quiet elevation and service rooms on the exposed one. Use the building form to shelter a courtyard, then ventilate bedrooms from it. Design dual aspect units so that a quiet aspect exists at all. Use winter gardens or acoustically screened balconies as a buffer that allows an openable inner window. Reduce glazing on the elevations with the worst solar gain rather than accepting a standard window type across the whole facade.

Each of those decisions is made in the first few weeks of a project and is close to impossible afterwards. Which is the real point: this is a massing and orientation problem that presents itself as a services problem.

PlanScope drafts noise and overheating statements structured to UK planning policy, with facts flagged where they need confirming, and councils expect a suitably qualified author for both. Browse the 66 report types to scope the rest of the submission.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.