Planning permission to split a house into two flats
There is no permitted development route for splitting a house into flats. Unit size, outdoor amenity space and where the bins and bicycles go are what decide the application.
Converting a house into flats planning permission is always required. There is no permitted development route and no threshold below which subdivision escapes the system, because section 55(3)(a) of the Town and Country Planning Act 1990 says that using as two or more separate dwellinghouses a building previously used as a single dwellinghouse involves a material change of use of the building and of each part of it. That wording is deliberate and it has no exceptions.
So the question is never whether you need permission. It is whether the two flats you want will meet the standards the authority applies, and on a typical two-storey terrace or semi the binding constraints are unit size, outdoor amenity space, and where the bins and bicycles go.
Why subdivision is always a material change of use
The statutory provision puts subdivision beyond argument. It also has two consequences people miss.
The change of use applies to each part of the building, so both flats are new planning units, and the lawful use of the original house does not carry forward to either of them as a right to revert.
And the application is not a householder application. Householder applications cover alterations to a single dwellinghouse, and a proposal creating flats falls outside that category, so it is a minor application with the corresponding fee. Get the application type right at the start, because submitting on the wrong form causes an invalidation and loses a fortnight.
A Planning Statement is worth preparing even on a small scheme, because the case is a policy case rather than a design one.
Unit size and the space standard
Most authorities have adopted the nationally described space standard through their local plan, which makes it a policy requirement rather than guidance. Where it has been adopted, the figures are not negotiable.
| Unit | Bedspaces | Minimum gross internal area |
|---|---|---|
| One bedroom, one person | 1 | 39 m2, or 37 m2 where a shower room replaces a bathroom |
| One bedroom, two persons | 2 | 50 m2 |
| Two bedrooms, three persons | 3 | 61 m2 |
| Two bedrooms, four persons | 4 | 70 m2 |
Alongside the areas, the standard sets minimum bedroom sizes of 7.5 m2 for a single and 11.5 m2 for a double, and requires a floor to ceiling height of at least 2.3 metres over a stated proportion of the area. It also requires built-in storage, which is the item most often designed out and then asked for.
Check whether your authority has adopted the standard, and if it has not, check what its local plan says instead. Some authorities operate their own areas, and a few apply nothing beyond a general amenity policy.
The practical point on a narrow terrace is that two flats of 50 m2 each need 100 m2 of internal area plus circulation, and many two-bedroom terraces do not have it. A ground floor flat with the garden and a larger upper flat sometimes works better than an even split.
Amenity space, bins and cycles
These three items refuse more subdivision applications than anything else, and all three are solvable at design stage.
Outdoor amenity space policies vary. Some require private space for each flat, some accept a shared garden, and most require the ground floor flat to have direct access to private space. A first-floor flat with no outdoor space at all is acceptable in some authorities and not in others, so read the policy before drawing the plan.
Bin storage has to be within the carry distance the council's waste service specifies, screened, and large enough for the containers actually issued for two households, which is roughly double what the house currently has. Draw the containers to scale rather than labelling a rectangle.
Cycle storage should be secure, covered, at ground level and accessible without carrying a bicycle through a flat. One space per flat is a common minimum and two is often required for a two-bedroom unit. A Waste Management Strategy covers the refuse side in more detail on larger schemes.
Presenting all three on a single annotated site plan, with dimensions, answers the objection before it is written.
Parking stress evidence
Two flats generate more car ownership than one house in most locations, and the highway objection is about displaced on-street parking.
Where the site is in a controlled parking zone, the answer is usually a restriction on permit eligibility secured by obligation, and the authority will want to know the flats are marketed as permit-free. Where there is no zone, expect to be asked for parking stress evidence.
The accepted method surveys occupancy of the surrounding streets overnight midweek and at a weekend, avoiding school and public holidays, counting parked vehicles against available kerb length within a defined walking distance. A Parking Survey carried out that way, with the survey dates and the extent surveyed stated, is evidence. A statement that there is usually space in the street is not.
Do the survey before you buy if parking is tight, because it is the cheapest piece of due diligence available on a conversion.
Sound insulation and the policy hook
Separating floors and walls between flats have to meet the airborne and impact sound requirements in the Building Regulations, and in a conversion that usually means a new floating floor build-up, which eats ceiling height in the ground floor flat.
The planning hook is residential amenity, and authorities often condition an acoustic scheme. Conditions must not duplicate controls exercised under other regimes, so a condition simply requiring compliance with the Building Regulations is objectionable. A condition requiring a pre-completion sound insulation test and the submission of results is a different matter and is commonly accepted.
Deal with it in the design rather than leaving it to the contractor. A conversion where the ceiling height falls below the space standard minimum after the acoustic floor is installed has failed two policies at once. A Noise Impact Assessment is normally only needed where an external source such as a road or commercial plant affects the flats.
Loss of family housing
A growing number of authorities have policies resisting the loss of larger family homes, usually defined by bedroom count, in areas where they are in short supply. Some apply the policy only above a stated floor area, some only in defined areas, and some operate a presumption against subdivision of any house with three or more bedrooms.
This is the policy that catches investors who have run the numbers on the units and not on the plan. Find it before exchange. Where the policy exists, the arguments that sometimes succeed are that the house is unsuitable for family occupation as it stands, that the mix in the immediate area is already predominantly flats, and that the proposal retains a family-sized unit as one of the two flats.
Assembling the application and the conditions to expect
The pack for a straightforward two-flat conversion:
- Existing and proposed floor plans and elevations, at consistent scales, with room areas and the gross internal area of each flat annotated.
- A site plan showing bin and cycle storage with dimensions, amenity space allocation and any parking.
- A Design and Access Statement where the site is in a conservation area and the threshold is met.
- A short planning statement addressing unit sizes, amenity, storage, parking and any family housing policy.
- Parking stress evidence where provision is reduced.
- Ownership certificate and, for flats, the correct application type and fee.
The conditions to expect are the bin and cycle stores to be provided before occupation, the amenity space to be laid out as approved, an acoustic testing requirement, obscure glazing to any new window causing overlooking, a restriction on further subdivision, and sometimes a permit-free obligation.
And do not build first. A subdivision carried out without permission is unauthorised development in respect of each flat, and while it may eventually become immune, the enforcement time limits have been changed by recent legislation and the position should be checked rather than assumed. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Can I split a house into two flats under permitted development?
No. Section 55(3)(a) of the Town and Country Planning Act 1990 provides that using a former 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 right and no size threshold below which the requirement falls away.
Is a subdivision a householder application?
No. Householder applications cover alterations to a single dwellinghouse, and a proposal creating flats falls outside that category, so it is submitted as a minor application with the corresponding fee. Using the wrong application type usually causes an invalidation and costs a fortnight, so confirm the category before you submit.
What minimum size does each flat have to be?
Where the authority has adopted the nationally described space standard through its local plan, a one bedroom one person flat needs 39 m2, or 37 m2 with a shower room instead of a bathroom, a one bedroom two person flat 50 m2, a two bedroom three person flat 61 m2 and a two bedroom four person flat 70 m2, with minimum bedroom sizes alongside.
Do both flats need private outdoor space?
It depends on the local plan. Some authorities require private space for each flat, some accept a shared garden, and most expect the ground floor flat to have direct access to private space. A first floor flat with no outdoor space is acceptable in some areas and not in others, so read the policy before fixing the layout.
What parking evidence will the council ask for?
Where provision is reduced, a parking stress survey of the surrounding streets carried out overnight midweek and at a weekend, avoiding school and public holidays, counting parked vehicles against available kerb length within a defined walking distance. In a controlled parking zone the usual answer is a permit-free restriction secured by planning obligation instead.
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.