Subdividing a house into two flats: layout standards

Turning one house into two flats is development whatever the building looks like afterwards. Section 55(3)(a) of the Town and Country Planning Act 1990 treats the use of a single dwellinghouse as two or more separate dwellinghouses as a material change of use, so a full planning application follows even when nothing changes outside. Most arguments about house to flats conversion standards are settled by that single line before anyone opens a drawing.

What the council then judges is the quality of the two homes you have created and the effect of two households where there was one. Space, daylight, sound, bins, bikes, access and parking. Those seven headings account for the overwhelming majority of refusals on conversions.

Why subdivision always needs permission

There is no permitted development right to split a house into flats. Part 1 of the General Permitted Development Order 2015 gives rights to dwellinghouses, and a building containing flats is not a dwellinghouse for those purposes, so the rights fall away on the day the conversion is complete. The prior approval routes that landlords reach for, Class MA for commercial to residential and Class Q for agricultural buildings, do not cover a house at all.

A lawful development certificate confirms what is already lawful. It cannot authorise a subdivision you have not yet carried out unless the use is somehow already established, which for a family house it will not be.

Building control runs alongside and separately. Creating two flats from one house is a material change of use under the Building Regulations, which pulls in fire safety, sound insulation and, in many cases, ventilation and energy requirements that the existing house never had to meet.

Space standards and what counts as a compliant unit

The Nationally Described Space Standard sets minimum gross internal areas for new homes: 39 m² for a one bedroom, one person flat, 50 m² for one bedroom and two people, 61 m² for two bedrooms and three people, and 70 m² for two bedrooms and four people. A one bedroom, one person flat may drop to 37 m² where a shower room replaces a bathroom.

The standard is optional. A council can only apply it where it has adopted it as a policy in its local plan, with evidence of need and viability, and national guidance is explicit about that. Check the adopted plan before you assume either way, because some authorities apply their own standards instead and a few apply none.

Two numbers inside the standard catch conversions more often than the headline areas. Floor to ceiling height must be at least 2.3 m across three quarters of the gross internal area, which is a live issue once you add an acoustic floor build-up. And a double bedroom needs 11.5 m² with a minimum width of 2.75 m, a single 7.5 m² at 2.15 m wide. A generous Victorian front room subdivided lengthways often fails the width test while passing on area.

Room sizes, aspect and daylight

Aspect does more damage to conversion schemes than raw floor area. A lower ground floor flat lit only from a light well at the front, with a kitchen against the party wall, will struggle against any modern design policy, and officers say so bluntly in delegated reports.

Daylight in new dwellings is assessed against BRE guidance and BS EN 17037, which work on illuminance and daylit area rather than the old single average daylight factor figure. If you are digging out or enlarging a lower ground floor to make the unit work, the structural and drainage questions covered in our guide to basement extensions under an existing house apply in the same way to a garden flat.

Single aspect north facing units attract objection in London and in any borough that has copied the London Plan approach. That does not make them refusable everywhere. It does mean you should show, with plans rather than assertion, where the light comes from in every habitable room.

Sound insulation between the two flats

Approved Document E applies to a material change of use, and conversions generally have to be proved by pre-completion sound testing rather than by adopting a standard construction detail. The separating floor between the two flats is the difficult element in a nineteenth century house: joists often run into the party wall on both sides, so flanking transmission defeats an otherwise well built floor.

Planning and building control diverge here. The council's environmental health officer may also ask for internal noise levels to be demonstrated where the house faces a busy road or a railway, using the internal guideline values in BS 8233 for living rooms and bedrooms.

Solve the acoustics on section drawings, not in a note. A 150 mm floor build-up that takes the upper flat below 2.3 m of clear height is a planning problem and a building control problem at the same time.

Refuse, cycle storage and outdoor amenity

Two households produce two sets of bins, and the front garden is where they end up unless the drawings say otherwise. Councils publish their own storage and collection requirements through the waste service, and most set a maximum drag distance from the store to the collection point. Get the actual bin sizes from that page and draw the store to fit them.

Cycle storage is now asked for on conversions as routinely as on new build. Secure, covered, and reachable without carrying a bike through a flat.

Outdoor amenity is where honest schemes and optimistic ones separate. Splitting a rear garden gives the ground floor flat something usable and leaves the upper flat with nothing. Some authorities accept that in a town centre with public open space nearby. Others require a balcony, a terrace or a shared garden with defined access. The adopted local plan policy will say which, and the answer changes across a borough boundary.

Separate access and the shared entrance hall

The entrance arrangement carries more weight than its size suggests. A shared hall with two front doors off it reads as a converted house. A new external door punched through a side elevation, with an external staircase to a first floor flat, usually reads as harm, particularly in a conservation area or on a uniform terrace. Where external alterations are unavoidable, the reasoning in our note on matching materials to the existing house is what officers expect to see addressed.

Fire escape shapes the plan too. The common route out has to be protected, which tends to mean fire doors, a protected stair and no habitable room opening directly onto the escape route. Design that in at the start rather than discovering it after permission.

Parking, and the evidence that answers the objection

Highway objections to conversions rest on one claim: two households will park where one did. Answering it needs measurement rather than argument. A parking stress survey following the Lambeth methodology, counting occupied kerb space overnight on a midweek night and at a weekend, avoiding school holidays, is the evidence highway officers recognise. Where stress is below roughly 85 per cent of capacity there is usually room to absorb an extra car, though the threshold applied is a matter for the local authority.

As an illustrative example, a three bedroom terrace on a controlled parking zone street might be split into a two bedroom ground floor flat and a one bedroom flat above. The application would carry a Planning Statement setting the scheme against the adopted plan's conversion and amenity policies, a parking survey, floor plans showing gross internal areas against the space standard, and a bin and cycle store drawn to scale. Figures in that example are illustrative only.

A Design and Access Statement is not required for every conversion. Article 9 of the Development Management Procedure Order 2015 requires one for major development and for schemes in a conservation area or World Heritage Site that create one or more dwellings, so a two flat conversion inside a conservation area will need one.

Work the numbers before the drawings are finished. Gross internal area, ceiling height after the acoustic floor, bedroom widths, bin capacity, cycle spaces and overnight parking stress are all knowable at sketch stage, and every one of them is a refusal reason if left until validation.

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

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