Converting a house into flats: the planning permission route

Converting a house into flats needs a full planning application, every time. Section 55(3)(a) of the Town and Country Planning Act 1990 states that the use of a single dwellinghouse as two or more separate dwellinghouses involves a material change of use, so house to flats planning permission is required even where you touch nothing structural and add no floorspace. There is no prior approval route and no permitted development class that covers subdivision. If someone tells you a two-flat split can be done under permitted development because the external appearance is unchanged, they are wrong, and a lawful development certificate application would say so.

That single legal point shapes everything else. Because you are applying for a change of use plus works, the council assesses the scheme against its housing policies, its amenity standards and its highways standards, not just its design guidance. Refusals on flat conversions rarely turn on how the front elevation looks. They turn on unit sizes, refuse and cycle storage, outdoor space and parking.

Subdivision is development whatever the building work

The application type is usually "full planning permission" for a change of use and associated alterations, not a householder application. That matters for two reasons. Householder applications are exempt from the biodiversity gain condition; a flat conversion is not, so you have to engage with biodiversity net gain even if the answer is that the statutory exemption for development below the de minimis threshold applies. And the fee is calculated differently. Planning fees are set nationally and indexed, so check the current schedule before you submit rather than reusing last year's figure.

A second, quieter trap is the planning unit. If the house has already been used as a house in multiple occupation, or has an annexe with its own front door, the lawful use you are converting from may be arguable. Get that straight in writing at pre-application stage, because the description of development on the form has to describe the change accurately.

Creating self-contained flats also brings the Community Infrastructure Levy into play where the council has a charging schedule. Existing lawful floorspace that has been in use for at least six months in the previous three years can normally be offset against the chargeable area under the CIL Regulations 2010 as amended, which is why an empty, long-vacant house sometimes produces a surprise bill and an occupied one does not. Submit the CIL additional information form with the application and claim the offset with evidence of occupation.

What the local plan says about losing a family house

Many adopted local plans resist the loss of family-sized housing, usually expressed as a policy protecting dwellings above a floorspace or bedroom threshold, or one that only permits subdivision where the property is above a stated size. Others take the opposite line and support conversions in areas of high housing need. There is no national rule here, so the first thing to read is the housing chapter of the adopted local plan and any supplementary planning document on residential conversions.

Section 38(6) of the Planning and Compulsory Purchase Act 2004 means the plan is the starting point for the decision. If a policy says subdivision is only acceptable for houses over, say, a given gross internal area, arguing that the flats are well designed will not rescue an undersized house. Check the policy wording, measure the existing gross internal area properly, and if you fall short, price the appeal risk before you buy.

A linked point that catches investors: some plans require a mix of unit sizes in conversions, or resist all-studio schemes. Two one-bedroom flats often read better in policy terms than a studio plus a three-bedroom maisonette, or worse, depending on the borough.

Internal space, layout and the standards officers measure

The Nationally Described Space Standard (2015, updated 2016) only applies where a council has adopted it through a local plan policy, but in practice most urban authorities have. It sets minimum gross internal areas by bedrooms and bedspaces, minimum bedroom sizes and widths, built-in storage minimums and a floor-to-ceiling height of at least 2.3m over at least 75 per cent of the gross internal area.

Figures are from the government's technical housing standards; check the current published table and your council's own policy, which may add requirements on dual aspect, outlook or bedroom width.

The 2.3m height rule is what kills basement and loft flats. A converted cellar with 2.1m clear headroom under the joists cannot be counted as compliant habitable space without lowering the slab, and lowering the slab in a Victorian terrace brings underpinning, party wall procedures and a basement design that the structural engineer needs to see before the plans are drawn.

Officers also look at layout quality that the standard does not quantify: whether a bedroom window faces a blank wall a metre away, whether a habitable room is lit only by a rooflight over a shower room, whether the entrance hall to the upper flat swallows the ground-floor flat's living space. Draw the furniture. A plan showing a bed, a wardrobe and a circulation route answers the officer's question before it is asked.

An illustrative example

Take a 1930s three-bedroom semi of roughly 95 m² over two floors, being split into a two-bedroom ground-floor flat and a one-bedroom first-floor flat. On the numbers above, a 2-bedroom 3-person unit needs 61 m² and a 1-bedroom 2-person unit needs 50 m², totalling 111 m² before circulation. The scheme does not fit without a rear extension or a change of mix, most likely to two one-bedroom flats. This example is illustrative; measure your own building.

Bins, bikes and outdoor space

Refuse storage is the single most common reason a flat conversion gets a holding objection. Each flat needs its own bin capacity for residual waste, recycling and food waste, stored where residents can reach it and the collection crew can pull it out. Councils apply their own waste storage guidance, often drawing on BS 5906:2005 for carry and drag distances, and many refuse to collect from bins stored in rear gardens with no side access. Show the bin store on the block plan with dimensions and the bins drawn to scale, not a hatched rectangle labelled "refuse".

Cycle parking is next. Secure, covered, ground-level storage is expected for each flat. In London, London Plan Policy T5 sets long-stay minimums of one space for studios and one-bedroom homes and two for larger dwellings, with short-stay visitor provision on top. Outside London, standards vary by council; two Sheffield stands in a shared yard rarely satisfies a policy that asks for enclosed storage.

Private amenity space is the third of the trio. London Plan Policy D6 expects a minimum of 5 m² of private outdoor space for one and two-person dwellings, with an additional 1 m² for each extra occupant, and a minimum depth and width of 1.5m. Elsewhere, look for a garden-size or amenity-space standard in the local plan or a householder design guide. Where the rear garden is retained for a ground-floor flat, say clearly in the drawings and the statement how the upper flat is served, whether by a balcony, a terrace or a defined share of the garden with its own access. Undefined "communal" garden with no separate access from the upper flat is a recurring refusal reason.

Parking evidence when there is no off-street space

Two flats generate more car ownership than one house in most locations, and highways officers will say so. Where the street is uncontrolled, the counter-evidence is a parking stress survey. The widely used method, the one referred to in London appeals as the Lambeth methodology, counts kerbside capacity and parked vehicles within a defined radius of the site on two midweek nights and one weekend night in the small hours, outside school holidays. Occupancy above roughly 85 per cent is generally treated as a street at capacity.

If the street sits below that, you have a case that on-street demand can be absorbed. If it sits above, expect the officer to ask for a car-free or car-capped scheme, secured by a planning obligation or, in a controlled parking zone, by a condition preventing permit eligibility. That is only enforceable where the council operates permits, which is why controlled zones are easier to convert in than free-for-all suburban streets.

Do not commission the survey in August or over Christmas. The data will be challenged, and rightly.

The document pack and who writes what

Every council publishes a local validation checklist, and the flat-conversion requirements sit in it. The typical bundle for a two or three-flat scheme is:

1. Application forms, ownership certificate and the correct fee.
2. Location plan at 1:1250 with the site edged red, and a block plan showing bin and cycle stores.
3. Existing and proposed floor plans and elevations at 1:100 or 1:50, with a schedule of gross internal areas per flat.
4. A Planning Statement addressing the housing policy test, space standards, amenity and parking.
5. A Design and Access Statement where the site is in a conservation area or a World Heritage Site and the proposal involves one or more dwellings, as required by Article 9 of the Development Management Procedure Order 2015.
6. A parking stress survey or transport statement where highways impact is contested.
7. A heritage statement for a listed building or a conservation area site, plus listed building consent if internal fabric is affected.
8. A flood risk assessment if the site is in Flood Zone 2 or 3, which matters if you are putting a habitable room at basement level.
9. Biodiversity net gain information: either the statutory metric and a gain plan route, or a reasoned statement that a statutory exemption applies.

Some of these need a suitably qualified author. The biodiversity metric is expected to be completed by a competent person under the Environment Act 2021 regime, and any protected species work, including bat surveys triggered by a loft conversion in an older roof, must be done by a licensed ecologist. A drafting tool can structure the narrative and cite policy; it cannot survey a roof void. You can see all 66 report types by category if you want to check which of them your site is likely to trigger.

Building Regulations decisions that change the planning drawings

Planning and Building Regulations are separate consents, but on a flat conversion the Building Regulations drive the plan form, so ignoring them produces drawings you cannot build.

Approved Document B sets the escape strategy. A two-storey conversion usually needs a protected entrance hall and stair, 30-minute fire doors and either a protected route or an alternative escape from the upper flat, sometimes with a sprinkler or misting system where the geometry does not work. That protected hall eats floor area and has to be shown on the planning plan, because adding it later can push a unit below the space standard.

Approved Document E governs sound insulation between the new dwellings. For dwellings formed by a material change of use, the performance targets are less demanding than for new build but still real: a minimum airborne value of 43 dB DnT,w + Ctr for separating walls and floors, and a maximum impact value of 64 dB L'nT,w for separating floors. Meeting that in an existing timber floor usually means a resilient floating floor and an independent ceiling, which adds build-up depth and reduces ceiling height. Check the height against the 2.3m planning threshold before you finalise the section drawing.

Stacking follows from both. Put bedrooms over bedrooms and wet rooms over wet rooms where you can, keep the new bathroom away from the neighbouring party wall, and route the upper flat's soil pipe where it does not need a rear elevation change that triggers a further amenity objection.

One last sequencing point. Order the measured survey first, then the space-standard test, then the fire and acoustic strategy, then the planning drawings. Every conversion I have seen go wrong went in the other direction.

Enter your postcode on PlanScope to see which reports your conversion is likely to need before you commission anything.

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. For specialist technical reports, qualified professional input may be required.