Walkthrough: flat conversion of a large Victorian house

Subdividing a house into flats always needs planning permission. Section 55(3)(a) of the Town and Country Planning Act 1990 states that the use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse involves a material change of use, so there is no permitted development route and no prior approval shortcut. What decides the application is rarely the principle. It is unit sizes, sound insulation, refuse and cycle storage, and whether the street can absorb the parking.

The walkthrough below is illustrative. The building, the unit mix and the survey results are invented to show the sequence and the evidence, not taken from a real application.

The building and the proposed unit mix

An 1890s four storey end of terrace, about 320 m² of internal floor area over basement, ground, first and second floors, with a 12 metre rear garden and no off street parking. Lawfully a single dwellinghouse in Class C3, last used as a family home, in an outer London borough with a controlled parking zone on adjoining streets but not on this one.

The proposal is four self contained flats: a two bedroom flat at basement and ground with garden access, a one bedroom flat at ground and mezzanine, a two bedroom flat at first floor, and a one bedroom flat in the roof and second floor. No extension, no dormer, no change to the front elevation.

That last point is worth aiming for on a first conversion. A scheme with no external alterations removes design, daylight and neighbour amenity objections at a stroke, and leaves the officer with the internal standards to consider.

Subdivision as a material change of use

The application is a full minor application for a change of use and associated works, not a householder application, and the fee is calculated per additional dwelling rather than as a flat householder fee.

Two local policy layers usually apply on top of the national position. The first is a conversion policy: many authorities permit subdivision only where the property exceeds a stated floor area, or where it is not a family sized house in an area with a shortage of them. The second is a mix policy, setting expectations about the proportion of larger units. Read both before you fix the layout, because a scheme of four one bedroom flats in a borough protecting family housing will struggle whatever its internal quality.

Also check whether an Article 4 direction applies. It would not affect this conversion, since Class C3 to Class C3 subdivision needs permission anyway, but it matters if the alternative plan is a house in multiple occupation, which is a different use class and a different application.

Space standards and room sizes

The Nationally Described Space Standard is the yardstick most authorities have adopted into their local plans. Where it is adopted, it is a policy requirement and a scheme that misses it is refused on that ground alone.

The figures that decide most conversions are the gross internal area minima: 39 m² for a one bedroom one person flat, reduced to 37 m² where a shower room replaces a bathroom, 50 m² for a one bedroom two person flat, 61 m² for a two bedroom three person flat and 70 m² for a two bedroom four person flat. Single bedrooms must be at least 7.5 m² and double or twin bedrooms at least 11.5 m². At least 75 per cent of the gross internal area needs a floor to ceiling height of 2.3 m or more, and built in storage counts towards the total up to a stated limit.

The 2.3 m height rule is what catches Victorian conversions. Basement floors and roof spaces both lose area to it, and the fourth flat in a scheme like this often fails on usable area once the sloping ceilings are measured properly. Draw the 2.3 m line on the section and colour the area that counts. Officers ask for exactly that drawing.

Sound insulation between flats

This is a Building Regulations requirement, and it is the one that costs money at construction stage.

A conversion is a material change of use, so Part E applies at the performance standards for conversions rather than for new build. Those standards are less demanding than the new build figures, at 43 dB airborne sound insulation between dwellings and 64 dB for impact sound through floors, compared with 45 dB and 62 dB for purpose built dwellings. The catch is the compliance route. The pre approved separating construction scheme that lets new build avoid testing does not cover a material change of use, so a conversion has to demonstrate compliance by pre completion testing.

Practically that means an isolated floor build up, usually an independent ceiling on resilient hangers with a floating floor above, losing 150 mm or more of ceiling height on each floor. Combine that with the 2.3 m requirement and you see why conversions are drawn in section before they are drawn in plan.

Planning and building control overlap here. Sound insulation is a building control matter, but amenity policies let a council ask for an acoustic design statement at application stage, and some authorities condition pre completion test results before occupation. Where external plant or a commercial use sits below, a full Noise Impact Assessment may be needed as well.

Bin, bike and amenity provision

These three are the most common reasons a conversion is refused on an otherwise acceptable layout.

• Refuse and recycling storage for four flats, sited within a reasonable carry distance of the collection point, screened, and large enough for the borough's actual bin sizes rather than a generic box on the plan.
• Long stay cycle parking, typically one space for a one bedroom home and two for larger homes under London standards, in a secure covered store with genuine access rather than a hook in a passage.
• Private or shared amenity space. The ground floor flat takes the garden in most schemes, which leaves the upper flats with none, so check whether the local plan sets a minimum and whether shared access to the garden is required.

Draw these to scale, with the bins and the bikes shown as objects. A hatched rectangle labelled bin store gets a request for further information.

Parking stress and the survey used

With no off street parking and four flats replacing one house, the highway authority will want evidence that the street can absorb the demand. The standard evidence is a parking beat survey following the Lambeth methodology: counts of parked vehicles against measured kerbside capacity, taken in the early hours on a midweek night and a weekend night, across the street and a defined radius. Stress above roughly 85 per cent of capacity is generally treated as constrained.

A Parking Survey that shows spare capacity is often enough on its own. Where the survey shows stress, the responses are a car free condition or obligation, a commitment not to apply for permits where a controlled parking zone exists, and improved cycle provision. Surveys have to avoid school holidays and unusual weeks, so programme them rather than fitting them in.

Biodiversity net gain and the garden

Do not assume a conversion is exempt. Biodiversity net gain applies to planning permissions unless an exemption applies, and the de minimis exemption is narrow. If the works disturb more than a small area of habitat, or if garden ground is hard surfaced for bins and bikes, a metric baseline and a Biodiversity Net Gain Assessment will be needed, and the baseline has to be taken before anything is cleared.

Fire safety is worth a sentence too. A Fire Statement under the planning regime applies to relevant buildings of 18 metres or seven storeys and above, so a four storey conversion does not need one. Part B of the Building Regulations still governs the protected stair, escape windows and alarm system, and those requirements shape the layout more than any planning policy will.

The submitted pack and the conditions

For this illustrative scheme: forms and ownership certificates, location plan, existing and proposed floor plans, sections showing the 2.3 m line, elevations, a Planning Statement addressing the conversion policy and the space standard compliance unit by unit, a parking beat survey, a refuse and cycle storage plan, an acoustic design statement, and a biodiversity metric with a gain plan where required. A Design and Access Statement is not required for a minor application unless the site is in a conservation area or the building is listed.

Conditions to expect: pre completion sound insulation testing before occupation, provision of the refuse and cycle stores before first occupation, removal of permitted development rights for front hardstanding, obscure glazing to any flank window, and a car free restriction where parking stress was demonstrated. A mixed use variant of the same building type runs differently, and the walkthrough of retail below flats shows where the tests separate.

Enter the property postcode on PlanScope to see which of the 66 reports a conversion of this kind is likely to need before you commission anything.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. The scheme described is illustrative, and this is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional.