Subdividing a house into two self-contained flats

Converting a house into flats needs planning permission, always. Section 55(3)(a) of the Town and Country Planning Act 1990 says so directly: the use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse 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.

So the question is never whether to apply. It is whether the scheme will survive the four things that refuse these applications: a restrictive local conversion policy, undersized units, no usable amenity or bin and cycle provision, and parking.

Why subdivision is a material change of use

The use class does not change. A house is C3 and two flats are C3. What changes is the number of planning units, and the 1990 Act treats that as development in its own right.

That distinction matters practically, because people reason from the use class and conclude that nothing has happened. Two dwellings generate two households' worth of vehicle movements, two sets of bins, two lots of comings and goings, and two separately transferable interests capable of further change later. That is what the council is assessing.

It also means the description of development needs to be right: conversion of a dwellinghouse into two self contained flats, with whatever external works are proposed listed separately. Vague descriptions cause trouble at the condition drafting stage.

Local plan policies that restrict conversions

This is the part to check before you buy, and it is the part most often skipped.

Councils vary enormously here, and their policies fall into recognisable types:

• A floorspace or bedroom floor: conversion permitted only where the property exceeds a stated size, on the basis that smaller houses should stay as family homes.
• A concentration test: refusal where the proportion of flats in the street or a defined radius already exceeds a stated figure.
• A retention of family housing policy protecting houses with a stated number of bedrooms.
• A frontage or character test in areas where conversion has visibly eroded the street.
• Minimum unit size and amenity requirements applied specifically to conversions.

Read the adopted plan policy and any householder or conversion supplementary planning document, and read the actual wording rather than a summary. A policy that says conversions "will be resisted where" is a different proposition from one that says they "will not be permitted unless".

Where the property sits in a conservation area, the external alterations get an additional layer of assessment and a Heritage Statement will be needed. Article 4 directions are a separate matter and are aimed mostly at houses in multiple occupation rather than self contained flats, but check whether one covers the property anyway.

Minimum space standards and room sizes

Where the local plan has adopted the nationally described space standard, it operates as policy and it is applied strictly.

The figures that matter for a two flat conversion:

Built in storage is included within those areas: 1 m² for a one bedroom unit, 2 m² for two bedrooms. A single bedroom must be at least 7.5 m² and at least 2.15 m wide. A double must be at least 11.5 m² and 2.75 m wide. At least 75 per cent of the gross internal area needs a floor to ceiling height of at least 2.3 m, which is the figure that catches lower ground floors and converted roof spaces.

Run those numbers before drawing anything. A typical two storey terraced house of around 90 m² does not split into two compliant units unless it has a rear extension or a usable loft, and the scheme that fails is usually the one where the ground floor flat has been squeezed to 34 m² to give the upper flat a second bedroom.

Where the standard has not been adopted locally, the council will still apply some amenity judgement, and the space standard is the reference point everyone reaches for.

Private amenity space, bins and cycle parking

Three practical requirements that decide more applications than design does.

Amenity space. Most policies expect each flat to have access to some private or shared external space, with the garden usually allocated to the ground floor unit and a balcony, terrace or defined shared area for the upper one. A scheme that gives the whole garden to one flat and nothing to the other invites a condition or a refusal. Splitting a small garden badly is worse than sharing it well.

Refuse and recycling. Storage has to be within the curtilage, screened from the street, sized for the council's bin types and both flats' capacity, and within the carrying distance the council's standard sets for residents and for the collection crew. Bins stored in the front garden with no enclosure is one of the most common reasons a decent scheme gets a refusal on street character.

Cycle parking. Secure, covered and accessible without carrying a bicycle through a habitable room. One space per flat is the usual minimum, with more expected for larger units in urban authorities. A shed at the end of a garden that can only be reached through the ground floor flat does not serve the upper flat.

Show all three on the block plan, dimensioned. Officers cannot condition what they cannot see.

Parking and on-street pressure

Two flats generate more parking demand than one house in most locations, and the highway officer will say so.

Where the adopted standard requires off street spaces you cannot provide, the argument has to be made on evidence rather than assertion. That means a Parking Survey using an established methodology: overnight and weekend counts of on street occupancy in a defined radius, carried out outside school holidays, expressed as a stress percentage. Where stress is below the threshold the authority uses, the case is straightforward. Where it is above it, the options are to reduce the scheme, provide a space, or offer a car free arrangement secured by an obligation where the authority operates controlled parking.

Do not offer a car free unit in an area with no parking controls. It cannot be enforced and officers know it.

Sound insulation and building regulations

Planning permission does not deal with sound between the flats. Building Regulations do, and the requirement is stricter for conversions than most investors expect.

Approved Document E applies to a material change of use, so the separating floor and any separating walls have to meet the performance standards for airborne and impact sound. Conversions use the pre completion testing route, because the alternative certified construction route is available for new build only. That means a test on completed construction, and a failed test means opening up finished work.

Budget for the build up. A typical solution on a Victorian timber floor involves an independent ceiling, resilient layers and added mass, and it eats 150 mm or more of height. On a house with 2.4 m ceilings, that interacts directly with the 2.3 m planning height standard. Resolve the two together at design stage rather than discovering the conflict on site.

Also in scope: Approved Document B for means of escape, which usually requires a protected stairway, 30 minute separation and interlinked alarms; Part F for ventilation; and Part L for the elements you alter. A conversion that passes planning and fails Part B at building control is a common and avoidable outcome.

Access, entrances and the street frontage

The external appearance is what the neighbours see and what the case officer photographs.

What works:

1. Retain a single front door serving a shared hall wherever the internal layout allows, so the building still reads as a house.
2. Where two doors are unavoidable, put the second one on a side or rear elevation rather than the principal frontage.
3. Keep meter boxes, soil pipes and extract terminals off the front elevation.
4. Avoid external staircases to upper floors. They are refused more often than any other element of these schemes.
5. Keep the front garden as garden, with bin storage enclosed and planted rather than fenced in bare timber.

The pattern is consistent: schemes that keep the outside looking like a house get permission more easily than schemes that announce the conversion from the pavement.

Documents and drawings to submit

For a two flat conversion outside a designated area:

• Application form, ownership certificate and the correct fee.
• Site location plan at 1:1250 and a block plan showing bins, cycle storage, amenity space allocation and any parking.
• Existing and proposed floor plans and elevations at 1:50 or 1:100, with the gross internal area of each proposed flat annotated.
• A room by room schedule showing each bedroom's area and width against the space standard.
• A Planning Statement addressing the conversion policy, unit sizes, amenity, bins, cycle parking and parking stress.
• A Design and Access Statement where the property is in a conservation area or the works affect a listed building.

Annotating the areas on the drawings is the simplest thing you can do to speed up determination. An officer who has to calculate your floor areas will assume the worst. You can browse all the report types if you want to check whether anything else is likely to be asked for on your particular site.

This is general guidance, not legal advice. Conversion policies, space standards and parking standards are set locally, so 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.