Converting a house into flats: the policy tests
Using a single dwellinghouse as two or more dwellings is a material change of use with no permitted development route. The principle of subdivision fails more applications than the floor plans do.
Converting a house into flats always needs planning permission. 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 involves a material change of use. There is no permitted development route, no prior approval shortcut, and no threshold below which it does not apply.
House to flats planning policy then divides into two layers. Whether the council accepts subdivision in principle at that address, and whether the flats you have drawn meet its standards for size, light, outlook, amenity space and storage. Applications fail on the first layer more often than the second, and the first layer is the one you can check before you buy.
Why subdivision is always an application
The change of use test does not depend on external alterations. A conversion carried out entirely internally, with no change to the elevations, is still development.
That matters for two reasons. It means an internal conversion carried out without permission is a breach of planning control, with the immunity clock running only from when the use began. And it means that a building converted informally years ago may or may not be lawful, so the planning history and any certificate of lawfulness need checking before purchase.
External works usually follow anyway. A second entrance door, a new rear window to serve a bedroom, a flue, external stairs, bin and cycle stores and a subdivided garden are all part of the same application.
Council policies that restrict or resist conversion
Read the development plan before the floor plans. The recurring policy types:
- protection of family housing, resisting the loss of dwellings with three or more bedrooms or above a stated floor area
- a minimum property size or frontage width below which subdivision is not supported, which rules out most modest terraces
- concentration policies limiting the proportion of converted or non-family units within a street, a block or a stated radius
- policies requiring off-street parking to be retained, or resisting conversion where the street is in a controlled parking zone at capacity
- character policies in conservation areas resisting the visual effects of subdivision, particularly multiple doors, external stairs and front garden bin stores
- Article 4 directions removing permitted development rights for houses in multiple occupation, which are common in university towns and affect a related but different conversion route
Concentration policies are where applications quietly die. Where the policy caps converted units at a stated proportion within a defined radius, your own survey of the street is the evidence that decides it, and the council's records are often out of date.
Minimum unit sizes, outlook and daylight
Where a council has adopted the nationally described space standard through its local plan, it becomes the working requirement. The headline figures are a one bedroom one person flat at 39 square metres, or 37 with a shower rather than a bath, a one bedroom two person flat at 50, a two bedroom three person flat at 61, and a two bedroom four person flat at 70. Single bedrooms are 7.5 square metres and doubles 11.5, with minimum widths of 2.15 and 2.75 metres. Ceiling height should be at least 2.3 metres over at least 75 per cent of the gross internal area.
That last figure catches loft conversions and basement flats, and it is worth checking with a tape before drawing anything.
Outlook and daylight decide the basement and rear-of-plot flats. A habitable room lit only by a light well, a bedroom looking directly at a retaining wall 1.5 metres away, or a single-aspect north-facing flat at lower ground level will attract an objection from the officer even where the areas comply. Where light is marginal, provide the analysis rather than waiting to be asked.
Every flat needs a private entrance arrangement that works, and the hallway that creates is real floor area. Two flats sharing one front door with an internal common hall is usually a better plan than two doors on the front elevation, and it reads better in a conservation area.
Amenity space, cycle and bin storage
Most policies require some private or shared outdoor amenity space for each flat, with the size set locally. The usual arrangement gives the ground floor flat the garden and the upper flat either a share, a balcony or nothing, and a scheme that gives the upper flat nothing at all is worth checking against the policy before submission.
Bin storage is the detail that generates most officer comments on small conversions. Size the store for the containers the collection authority actually uses, multiplied by the number of flats, allow for recycling and food waste, keep it within the carry distance the waste team specifies, and screen it. A row of wheelie bins in a front garden in a conservation area is a refusal reason in its own right.
Cycle storage is now expected on almost every conversion. Secure, covered, accessible without carrying a bike through a flat, and at the ratio the local standard sets.
Include a site location plan at the right scale with the red line drawn around everything you rely on, including the bin store position and any land that will be subdivided.
Sound insulation and the Part E overlap
Sound insulation between flats is a Building Regulations matter, not a planning one, and officers should not be conditioning it. It still determines whether the scheme is buildable.
A material change of use to flats brings Part E into play. Conversions are tested by pre-completion sound testing rather than by the pre-approved construction details route, which is available only for new build, and the performance standards for conversions are less demanding than for new construction while still being difficult to meet in a Victorian house with 100 millimetre timber floors.
Budget for it properly. Independent ceilings, resilient floor treatments, isolating the joists from the party structure, and dealing with flanking transmission through the party wall and through service penetrations. The cost frequently exceeds the cost of the new kitchen and bathroom combined, and it is discovered late by owners who assumed it was a planning issue.
Illustrative example: a three-bedroom Victorian terrace converted into two flats. The ground floor flat takes 58 square metres with the garden, the upper takes 52 square metres with a small rear terrace above a back addition. Both meet the adopted space standard. The scheme is refused at first attempt on the loss of a family dwelling under a local policy, then approved on a second application evidencing that the property has been in flatted use for a decade. The details are illustrative.
Documents to submit for a two-flat conversion
The pack for a small conversion is short, and every item earns its place:
- site location plan and block plan, with the red line and any subdivided curtilage shown
- existing and proposed floor plans and elevations at a consistent scale, with room areas and ceiling heights annotated
- a planning statement addressing the policy on subdivision first and the standards second
- a design and access statement where the site is in a conservation area or the works affect the elevations
- bin and cycle store details, with capacity and dimensions
- parking and access details where a space is lost or an access altered
- daylight analysis for any basement or single-aspect habitable room
Check the authority's local validation list before submitting, because conversions attract local requirements more often than any other small application type.
On the levy, conversion of a building that has been in lawful use for a qualifying period before permission generally falls outside the charge, but the relief depends on the use history and on notices being served in the right order. Get the CIL forms and their timing right, because a missed notice can turn a nil charge into a real one.
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. Enter your postcode on PlanScope to see which documents a flat conversion is likely to need.
Frequently asked questions
Do I need planning permission to convert a house into two flats?
Yes, always. The Town and Country Planning Act 1990 treats use of a building previously occupied as a single dwellinghouse as two or more separate dwellings as a material change of use. It applies even where the works are entirely internal and the elevations are unchanged, and there is no permitted development or prior approval route.
Which policies most often refuse a conversion?
Policies protecting family housing, minimum property size or frontage thresholds below which subdivision is unsupported, and concentration policies capping the proportion of converted units within a street or radius. Parking policies in controlled parking zones and conservation area character policies also feature. Check all of these before buying rather than after drawing.
What minimum flat sizes apply?
Where a council has adopted the nationally described space standard, a one bedroom one person flat is 39 square metres, or 37 with a shower rather than a bath, a one bedroom two person flat is 50, a two bedroom three person is 61 and a two bedroom four person is 70. Ceiling height must be at least 2.3 metres over 75 per cent of the floor area.
Is sound insulation a planning matter?
No, it falls under Part E of the Building Regulations, and officers should not condition it. It still decides whether the scheme is buildable. Conversions are verified by pre-completion sound testing rather than by the pre-approved details route available for new build, and the cost of independent ceilings and resilient floors often exceeds a new kitchen and bathroom together.
Does a flat conversion attract the community infrastructure levy?
Conversion of a building that has been in lawful use for a qualifying period before permission generally falls outside the charge, but the relief depends on the use history and on the correct notices being served in the right order before works start. Missing a notice can turn a nil charge into a real liability.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.