Splitting one dwelling into two: the use position

Splitting a house into two flats needs planning permission every time. Section 55(3)(a) of the Town and Country Planning Act 1990 puts it beyond argument: the use of a building previously used 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. No size threshold, no de minimis, no permitted development class that covers it.

What varies is whether you get permission, and that turns on room sizes, bins, parking and the local plan's view on losing family housing.

Subdivision is a material change of use by statute

Section 55(3)(a) exists because the general test of materiality would otherwise invite argument. Parliament put subdivision beyond doubt.

The consequence is worth stating plainly. Even where the external appearance is unchanged, even where no works are needed beyond a lock on an internal door, the creation of two separate planning units from one dwelling is development requiring permission. Councils find out through council tax banding, through electoral roll data, through a tenant's complaint and through letting adverts.

The second half of the subsection matters too. The change of use affects each part of the building so used, which means the ground floor flat and the first floor flat are each unauthorised, and the enforcement position is not cured by reverting one of them.

A note on what counts as a separate dwellinghouse. The question is whether each unit is capable of independent occupation as a separate planning unit, judged on self-containment, separate access, kitchen and bathroom facilities and the degree of physical separation. Sharing a front door does not prevent two flats existing. Sharing a kitchen generally does.

No permitted development route exists

People look for one, because Part 3 of Schedule 2 to the GPDO 2015 has classes for almost everything else. It has nothing for subdividing a dwelling.

Class L permits movement between use as a dwellinghouse and use as a small house in multiple occupation, in both directions, which is a different thing entirely: one planning unit with shared facilities, not two self-contained units. Class MA deals with commercial to residential. Class Q deals with agricultural buildings. None of them touch C3 to two C3 units.

That absence is deliberate. The local impacts of subdivision, on parking, refuse, amenity and housing mix, are exactly the impacts councils are expected to assess case by case.

Room sizes and the space standard

The Nationally Described Space Standard is the first number an officer checks, and it only applies where the local plan has adopted it as policy. Check the plan before you design.

Where it does apply, the floor areas are fixed:

The standard also expects a floor to ceiling height of at least 2.3 m over 75 per cent of the gross internal area, which is where basement and top floor flats in Victorian houses fall over. A lower ground floor with 2.1 m of headroom under the joists will not meet it, and an officer who has measured a dozen of these will ask for a section drawing rather than take your word.

Where the plan has not adopted the standard, the policy test is usually a general one about adequate internal space and outlook, and the standard is still cited as a benchmark. Designing below it and arguing the plan is silent rarely ends well.

Outdoor space, bins and cycles

These three are the most common reasons a technically compliant subdivision gets refused, and all three are solved on the drawings rather than in the planning statement.

Private outdoor space requirements come from the local plan. London applicants should work to the London Plan standard of 5 m² of private outdoor space for one and two person dwellings with an increment per additional occupant. Elsewhere the standard is usually a garden depth or a proportion of plot area, and the ground floor flat taking the whole garden while the upper flat gets nothing is a frequent sticking point.

Refuse is the one people forget. Two flats need two sets of bins, usually four or six containers in total with recycling, and they need to be stored out of the front garden and wheeled to the kerb on a route under about 25 m. A bin store shown as a dashed rectangle on the site plan with no dimensions will come back as a request for details. Draw it to the actual container sizes the council collects.

Cycle parking for two units, secure and covered, is a standard requirement in most urban authorities now and sits awkwardly on a narrow terrace plot alongside the bins.

Parking and the local standard

Two units generate more parking demand than one, and in an unrestricted street that is a highway objection waiting to happen.

The defensible answer is evidence. Where the local standard asks for a space per unit and the site has one on street frontage, a parking stress survey of the surrounding streets measured over the relevant overnight period will tell you whether the demand can be absorbed. The Lambeth methodology, with surveys on a weekday night and a weekend night outside school holidays, is the convention most highway authorities recognise, and a parking survey carried out to it carries far more weight than an assertion about good public transport.

In a controlled parking zone the argument is easier, because the council can restrict permit eligibility by condition or agreement. In a zone with no controls and visible overnight stress, be honest with yourself about the prospects before paying the fee.

A short transport statement tying the survey, the cycle provision and the refuse route together is usually enough at this scale.

Sound insulation between the new units

Subdivision is a material change of use under the Building Regulations, which brings Requirement E1 into play for the new separating floor and any separating wall.

Approved Document E sets performance standards for dwellings formed by material change of use, and they are less demanding than those for new build: airborne sound insulation of at least 43 dB and impact sound transmission of no more than 64 dB, measured on site. Achieving them in a house with a single joist floor and no ceiling void is real construction work, not a layer of acoustic underlay.

This is a Building Regulations matter rather than a planning one, so it does not belong in the planning statement. It belongs in your cost plan, because the floor build up, the stair enclosure required by Approved Document B and the new front door arrangement frequently cost more than the kitchens.

Fire is the other half. Two flats off a common stair need a protected escape route, which usually means fire doors, a protected stair enclosure and interlinked detection to Approved Document B and BS 5839-6.

What to submit

A planning statement addressing the housing mix policy, the space standard position and the amenity provision. Existing and proposed floor plans and elevations at a proper scale. A site plan showing bin storage and cycle parking with dimensions. Sections through any lower ground or roof space room to demonstrate headroom. A parking survey and short transport statement where the street is under pressure. A design and access statement if the site is in a conservation area, since Article 9 of the Development Management Procedure Order requires one for residential development there.

The planning statement is where the housing policy argument lives, and on subdivision schemes that argument is the application. Many urban authorities have policies resisting the loss of larger family houses below a stated floor area, and if your house sits above that threshold you need to say why the scheme still accords with the plan.

The commercial equivalent raises a different set of tests, and the article on splitting a commercial unit into two covers those.

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.

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