Internal noise criteria in a Noise Impact Assessment

The internal noise criteria section of a Noise Impact Assessment is where the report stops describing the site and starts committing to numbers. For residential development in England it almost always sets internal ambient targets from BS 8233:2014, applies them room by room, and then has to show that the glazing and ventilation specification can actually deliver them. Get the criteria wrong at this point and every calculation downstream is measuring against the wrong line.

The section is short. It carries more weight than its length suggests, because environmental health officers read it first and check whether the rest of the report is consistent with it.

Which standard sets the internal targets

BS 8233:2014, Guidance on sound insulation and noise reduction for buildings, gives the internal ambient noise level guidance that councils in England expect for new dwellings. It is guidance rather than regulation, and it deals with steady ambient noise rather than individual events.

Alongside it sits the professional practice guidance on planning and noise for new residential development, published in 2017 by the Institute of Acoustics with the Association of Noise Consultants and the Chartered Institute of Environmental Health. That document sets the two-stage structure most assessments now follow: an initial site noise risk assessment, then a full assessment covering acoustic design, internal levels, external amenity and any other relevant issues.

Both work inside the policy framing of the Noise Policy Statement for England, which introduced the observed adverse effect levels that the NPPF and the Planning Practice Guidance on noise rely on. The criteria section should say which of these documents it is applying and why, not list all three and move on.

Daytime and night-time criteria by room

BS 8233 sets different targets for different room types and different periods. The section should present them as a table and then state which rooms in the proposed layout each figure applies to.

Individual events at night are handled separately. BS 8233 notes that regular individual noise events can cause sleep disturbance and points to a guideline of 45 dB LAmax,F inside bedrooms, which matters enormously on sites near a railway, a flight path or a service yard with night deliveries. A scheme beside a freight line can meet the 30 dB LAeq,8hr figure comfortably and still be unacceptable on pass-by maxima.

Studio flats need a decision. The report should say whether the combined space is assessed against the living room figure or the bedroom figure, and the honest answer is that the night-time bedroom criterion applies to the sleeping area regardless of what the room is called on the plan.

Ventilation with the windows closed

The awkward part. BS 8233 internal levels can often be met with windows partly open, which the standard treats as giving in the region of 10 to 15 dB of attenuation depending on the opening. Where the facade noise level is high enough that the windows have to stay shut, the criteria section has to say so explicitly and the design has to provide an alternative means of ventilation that meets Approved Document F.

That alternative is usually acoustic trickle ventilators for background rates, or mechanical ventilation with heat recovery on the noisier facades. Both cost money and both take space in the ceiling void, which is why this needs settling before the general arrangement drawings are frozen rather than after.

State the assumed window position for every calculation. An assessment that reports compliance without saying whether the window was modelled open or closed is not an assessment, and a case officer who has seen a few of these will ask.

The overheating collision

Approved Document O restricts a scheme's ability to rely on opening windows where noise would prevent occupants from doing so at night. So the acoustic solution of sealing the facade and the overheating solution of purge ventilation through openable windows pull against each other on exactly the same sites: urban, south-facing, near a busy road.

The criteria section should cross-refer to the overheating strategy rather than pretend the two are independent. Running a TM59 overheating assessment with the same window opening assumptions as the noise model is the only way to know whether both sets of criteria can be met by the same building. Where they cannot, the answer is normally mechanical cooling or a change to the massing, and that is a cost conversation to have at design stage. A separate Overheating Risk Assessment will usually be required on the same application.

External amenity area criteria

BS 8233 gives a desirable level of 50 dB LAeq,T for external amenity areas such as gardens and balconies, with 55 dB LAeq,T as an upper guideline, and it recognises that in higher noise urban settings this may not be achievable and a compromise will be needed.

The criteria section should identify which external spaces are being assessed. A communal courtyard, a roof terrace and a ground-floor private garden behave differently, and the report is stronger for saying that the balcony on the road facade will exceed 55 dB LAeq,T while the courtyard will not, than for quoting a site-wide figure nobody can locate.

Where the upper guideline cannot be met, say what compensates: a quieter communal space, a protected courtyard, proximity to public open space. Do not simply assert that the exceedance is acceptable.

Demonstrating the criteria are achievable

Numbers on a page prove nothing until the report shows the facade build-up that delivers them. The internal criteria section should feed directly into a specification table giving the required sound reduction index for each glazing type and each ventilator, by facade and by floor level, derived from the measured facade levels.

Illustrative example. A third-floor bedroom on a main road facade with a measured free-field level of 62 dB LAeq,8hr at night needs roughly 32 dB of facade attenuation to reach 30 dB LAeq,8hr internally, which in practice means a sealed unit with an acoustic interlayer plus an attenuated mechanical supply. The arithmetic here is illustrative and a real calculation must use the measured spectrum, the room volume and the glazed area.

Where the assessment relies on a source-based method for plant or commercial noise rather than road traffic, the rating penalties applied under the industrial and commercial sound standard are a separate exercise, and we have covered how those are derived in a piece on rating penalties in a BS 4142 assessment.

Conditions that secure the specification

The council will condition the outcome, not the report. Expect a condition requiring the glazing and ventilation scheme to be submitted and approved before the relevant works, and increasingly a post-completion verification condition requiring measured internal levels in a sample of the worst-affected units before first occupation.

Write the criteria section so that a condition can be drafted straight from it. That means stating the criterion, the period, the room type and the measurement position in terms an officer can lift into a condition without rewriting. Assessments that bury the targets in prose get conditions that do not match the design, and discharging those conditions later costs far more than the drafting time saved.

A PlanScope Noise Impact Assessment draft sets out this structure, cites the standards by edition, and marks every measured level as still to be confirmed until survey data is supplied. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and councils expect a noise assessment to be authored by a suitably qualified acoustician.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.