Converting a house into two flats: amenity and storage

A house to two flats conversion always needs planning permission. Section 55(3)(a) of the Town and Country Planning Act 1990 puts it beyond argument: using a building previously occupied as a single dwellinghouse as two or more separate dwellinghouses is a material change of use of the building and of each part of it. There is no permitted development route, no prior approval and no Article 4 question to check.

Which means the application is decided on the detail, and the detail that decides it is usually amenity, refuse and cycle storage rather than the principle of subdivision.

Why these applications fail

Officers rarely refuse a subdivision because they object to two flats. They refuse because the scheme gives the ground-floor flat the whole garden and the upper flat nothing, or because the bin store has been drawn as a dotted rectangle in the front garden with no dimensions, or because the second bedroom measures 6.8 m² and the local plan sets a floor.

Each of those is fixable at drawing stage and expensive to fix after a refusal. The pattern is the same across most authorities: a policy on internal space, a policy on external amenity space, a policy on waste and cycle storage, and a highways position on parking. Address all four explicitly in the Planning Statement rather than hoping the drawings speak for themselves.

Internal space and room sizes

Where a council has adopted the Nationally Described Space Standard into its local plan, the figures become a policy requirement rather than guidance. The standard sets a minimum gross internal area of 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. Single bedrooms must be at least 7.5 m² and 2.15 m wide, doubles at least 11.5 m² and 2.75 m wide. Floor to ceiling height must be at least 2.3 m over 75 per cent of the gross internal area.

Built-in storage counts towards the area, at 1 m² for a one-bedroom flat and 2 m² for two bedrooms. Leave it off the plan and you lose both the storage credit and the argument that the flat works.

The 2.3 m height test is the one that bites in conversions. Victorian and Edwardian ground floors clear it comfortably. Second floors in a converted roof, and lower ground floors with a lowered slab, often do not, and a mezzanine or dormer that creates floor area below 2.3 m adds to the denominator without adding to the numerator.

Check whether your authority has adopted the standard before designing to it. Some have, some rely on their own figures, and a handful apply neither, in which case the argument runs on general amenity policy and you need to make the case positively.

Private and shared external amenity space

Most local plans expect each new flat to have usable outdoor space, and most set a minimum area with a minimum dimension so that a 5 m² strip 0.8 m wide does not qualify. In London, policy D6 of the London Plan sets 5 m² of private outdoor space for one and two person dwellings with 1 m² added per additional occupant, and a minimum depth and width of 1.5 m.

The practical sequence for a terraced or semi-detached house is this. Give the ground-floor flat a private garden with direct access from a habitable room. Give the upper flat either a dedicated section of garden reached by a side path, a rear balcony, or a share of a communal garden with a defined boundary. Do not give the upper flat a nominal balcony over a neighbour's garden boundary without testing the overlooking position, because that is where balconies and terraces run into overlooking objections.

Where no outdoor space can be provided for the upper flat, say so, explain why, and point to the proximity of public open space. An honest shortfall with a reason attached survives better than a fudged plan.

Bin storage that a collection crew will accept

Two flats means two sets of containers: residual waste, dry recycling and food waste for each, plus garden waste where the service is subscribed. That is commonly six or more containers where there was previously two or three, and a standard 240 litre wheeled bin occupies roughly 0.6 m by 0.75 m in plan with clear space in front of it to move.

BS 5906:2005 is the usual reference for carry distances and store design, but the figures that decide your application are the collection authority's own. Most publish a developer guide giving the maximum distance residents may be expected to carry waste to a communal store, the maximum distance the crew will drag a bin to the vehicle, and whether the vehicle can reverse into the site at all. Find that document and draw to it.

Three details that officers look for:

• A store with a hard surface, a door wide enough for the largest container, and a fall or drainage point if it is enclosed.
• Screening from the street that is not a 1.8 m close-boarded box across the whole front garden, because the design officer will object to that even if the waste officer accepts it.
• A route from each flat's front door to the store that does not pass through the other flat's private amenity space.

Front-garden stores on terraces are usually acceptable when they are low, set behind a hedge or a brick wall, and detailed to match the house. The refusals come when the store is tall, central and visible along the whole terrace.

Cycle storage and where it goes

Cycle parking standards are set locally and in London by policy T5 of the London Plan, which expects one long-stay space for a studio or one-bedroom flat and two for a two-bedroom flat or larger. A secure, covered store that a resident can reach without carrying a bike through the flat is the standard the officer applies.

In a terrace that means either a rear store reached along a side passage, or a front-garden store sharing the enclosure with the bins. A lockable store for two bikes needs roughly 2 m by 1 m internally plus door clearance. Stacking bins and bikes in the same enclosure works provided the drawing shows both at the same scale with the doors open.

The common failure is specifying a Sheffield stand in a communal garden that the upper flat cannot reach without a key to the lower flat's side gate.

Sound insulation and parking

Sound insulation between flats is a Building Regulations matter rather than a planning one. Approved Document E applies to flats formed by a material change of use and sets a minimum airborne performance of 43 dB and a maximum impact performance of 64 dB for separating floors and walls in conversions, tested on completion unless a recognised standard construction detail is accepted instead.

Planning gets involved when the layout stacks a habitable room above a bedroom, or puts a kitchen directly over a bedroom. Where that happens, expect a condition requiring a scheme of acoustic insulation to be approved before occupation, and design the stack sensibly at the outset so the condition is easy to discharge.

Parking is the most local variable in all of this. And it is the variable that moves most between authorities. Some authorities apply maximum standards and will accept a car-free flat in a controlled parking zone. Others expect one space per flat and will ask for on-street stress evidence if you propose fewer. If the street has no controls and the local plan sets a minimum, a parking survey is the only evidence that moves the highway officer.

Putting the application together

Two statements do most of the work. A Planning Statement that takes each relevant policy in turn and shows compliance or explains the departure, and a Design and Access Statement that covers the external alterations, the bin and cycle enclosures and the access arrangements for each flat.

Draw the stores to scale on the block plan with dimensions and container sizes annotated. Mark the carry route. Schedule the room areas and the gross internal area of each flat on the floor plans. Those three additions resolve most of the questions a validation officer or case officer would otherwise raise, and they take an afternoon.

Where the building sits above a commercial unit the analysis changes, because the host building is not a dwellinghouse and different policies on residential amenity and servicing apply. That situation is covered separately in the note on extending a flat above a shop.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. To see which documents a subdivision at your address is likely to need, enter your postcode on PlanScope. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.