Sound insulation between a commercial unit and flats above

The objection that stops most shop-to-flat conversions is not parking and it is not daylight. It is sound insulation between the retained commercial unit and the new flats above, and it is usually raised by the environmental health officer rather than by a neighbour. The Building Regulations minimum for a conversion is 43 dB airborne and 64 dB impact between dwellings, and those figures are nowhere near enough where the space below is a restaurant, a takeaway, a gym or a bar.

So the application has to do two things the applicant did not budget for: establish what the existing floor actually achieves, and set a design target for what it needs to achieve.

Why this is the objection that stops conversions

A first floor over a shop is usually a timber joist floor with lath or plasterboard below and boards above. It was built to keep a stockroom separate from a shop, not to keep a family separate from an extraction fan.

Environmental health officers know this, and they also know that once the flats are occupied the complaints land on them. If the insulation fails, the council's realistic options are to serve notice on the commercial tenant, who was there first, or to live with a nuisance it authorised. Neither is attractive, which is why the officer's starting position is often a holding objection until a scheme is demonstrated.

National policy supports them. The agent of change principle means the party introducing the noise sensitive use carries the cost of mitigation, and existing businesses should not face unreasonable restrictions because a residential use arrived above them later. That principle is why the answer to "the tenant will just have to turn it down" is no.

Airborne and impact performance are separate problems

These are two different transmission paths and they need two different design responses.

Airborne sound is speech, music and machinery noise travelling through the air, then through the structure, then into the air above. It is controlled by mass, isolation and absorption, and measured as a weighted level difference with the low frequency adaptation term, written DnT,w + Ctr. The adaptation term matters here more than anywhere else, because commercial noise below a flat is usually bass heavy.

Impact sound is energy put directly into the structure: footfall, a dropped crate, a cellar cooler mounted on a joist, a compressor bolted to a wall. It is measured as a weighted impact sound pressure level, L'nT,w, where a lower number is better, and it is controlled by isolating the source or by breaking the structural path.

Those last two rows are deliberately not numbers. The target is derived from the source levels in the unit below and the internal noise levels you need to achieve in the flats, and any consultant quoting a standard figure without measuring the source is guessing.

What the existing structure will and will not achieve

Measure before you design. A field test on the existing floor, with a source in the commercial unit and a receiver above, tells you the starting point and how much improvement you have to find.

Existing timber floors over shops commonly test well below the conversion standard. Getting from there to a target suitable for a restaurant below means three things, and usually all three.

An independent ceiling below the existing joists, hung on resilient bars or on a separate frame, with mineral wool in the cavity and two layers of plasterboard of different thicknesses. A floating floor above, on a resilient layer, decoupled from the joists and from the perimeter walls. And attention to flanking paths: joist bearings built into the party wall, the common stair, service risers, the shopfront head, and any penetration for pipework or cabling.

The two constraints that decide feasibility are headroom and tenancy. An independent ceiling takes 100 millimetres or more out of the commercial unit's height, and the commercial tenant has a lease that may not permit it. Establish that before the acoustician designs a ceiling nobody can install.

Where the floor structure is concrete the arithmetic is kinder for airborne sound and often worse for structure-borne impact, because concrete transmits vibration efficiently. The answer there is isolating the source rather than thickening the separation.

Setting a target above the minimum

The design target comes from working backwards. Take the internal noise levels appropriate for habitable rooms, typically expressed as daytime and night time equivalent levels for living rooms and bedrooms with a separate limit on maximum levels at night, then subtract them from the measured or predicted source levels in the unit below.

Where the unit below is licensed for entertainment, a level based target is not enough on its own. Music breakout is assessed in octave or third octave bands down to the low frequencies, and the criterion is often inaudibility in bedrooms rather than a broadband level. The planning and licensing regimes then interact, and they do not always align, which is covered in planning control and licensing control of venue noise.

State the target in the Noise Impact Assessment as a number with the metric, not as a description. "Suitable sound insulation will be provided" is the sentence that generates a second round of consultation.

Plant, refrigeration and deliveries

The floor is only half the problem. In a retained commercial unit the noise sources that reach the flats are often not in the unit at all.

Cellar cooling and refrigeration run continuously and produce low frequency structure-borne noise, which no ceiling will fix if the compressor is bolted to a shared wall. Kitchen extract ducts rising past bedroom windows carry both noise and, separately, odour, which is why the Ventilation and Extraction Statement and the noise assessment should be written as a pair, along with the extract termination height and odour control selection.

Deliveries and waste collection are the third source, and the one most often conditioned. Early morning bottle collections and refuse lifts produce short, loud events that wake people, and they are controlled by hours conditions rather than by construction.

Survey all of it before submission. An application that deals only with the separating floor will be conditioned on everything else, in wording somebody else has drafted.

Test based conditions and what they should say

For a conversion, pre-completion sound testing is required under the Building Regulations in any event, because the pre-approved construction details route applies only to new build. Planning conditions often add to that.

A workable planning condition names the target values with their metrics, names the separations to be tested and the number of tests, requires the results to be submitted and approved before occupation of the affected flats, and requires remedial works and retesting where a test fails. It is worth asking for the sample to be defined, because a condition requiring every separation to be tested on a large conversion is disproportionate and expensive.

Two drafting traps. A condition requiring compliance with Approved Document E adds nothing, because that already applies and its figures are too low for this situation. And a condition requiring approval of a scheme "in accordance with a scheme to be submitted" with no performance figures leaves the target to be argued about after the ceiling is built.

Management measures and their limits

Hours restrictions, delivery windows, a noise management plan for the commercial unit and a requirement to keep doors closed during amplified music are all legitimate planning conditions, and they do real work.

What they cannot do is control the lease. A planning condition binds the land, so it binds the commercial occupier too, but it is enforced by the council rather than by you, and it will not help with a tenant who complies with the condition while still being audible. Construction is the reliable control and management is the supplement.

Evidence to submit with the application

For a conversion above a retained commercial unit the pack that avoids a holding objection contains:

1. A field test report on the existing separating structure, with the test method and positions stated.
2. Source noise measurements or a source characterisation for the use below, including low frequency content where music or refrigeration is involved.
3. Stated internal noise targets for the new flats, with the metrics.
4. Proposed construction build-ups with laboratory test data for each element, and a treatment of flanking paths.
5. A plant and services noise assessment covering existing and proposed equipment.
6. An outline testing regime, with the separations and sample size proposed.

This is general guidance, not legal advice. Acoustic requirements for conversions above commercial premises are set case by case, so confirm the targets with the environmental health team and a qualified acoustician before relying on any figure here.

Enter your postcode on PlanScope to see which reports a change of use above a commercial unit is likely to need. 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.