Where the money goes
Two flats from one house cost three kinds of money. The first is design and approval: our £945 drawings + £150 survey (£1,095 total), the council’s fee and building control. The second is the build, which is most of the total. The third is the running cost of two homes instead of one.
We cover the first. The drawings fix the layout, the entrances, the stair and the fire separation, and that is where the build cost is decided, so it pays to draw the layout properly before a builder prices it.
Why it is planning permission, and what it costs
Section 55 of the Planning Act says that the use as two or more separate dwellinghouses of any building previously used as a single dwellinghouse involves a material change of use. Permitted development for houses does not stretch to that, so you apply.
The council fee for this category in the national fee schedule is charged for each additional dwelling. We quote the exact figure for your project at the start, so you can set it beside our fee.
Space: the standard each flat is judged against
Councils that adopt the national space standard judge a new flat against it. Its minimum floor areas for one-storey homes are 39 square metres for a one-person flat, 50 for a one-bedroom two-person flat, 61 for a two-bedroom three-person flat and 70 for a two-bedroom four-person one. It also asks for a ceiling height of 2.3 metres over at least 75% of the area.
A modest terrace on two floors does not always divide into two flats that each pass. The survey gives the real floor areas, so we know before you spend whether two flats work, or whether a flat and a maisonette would.
Fire and the old floors
Fire is the second test. For a converted dwellinghouse, Approved Document B asks for a review of the existing construction, because retained timber floors can make fire resistance hard to achieve. In a converted building of up to three storeys, REI 30 may be accepted for the structure if the escape routes follow Section 3.
For a single-storey flat, Section 3 lets the layout meet one of three approaches, including a protected entrance hall, or a plan where no habitable room is more than 9 metres from the flat entrance door and the cooking is away from the escape route.
Levy is the last item. A conversion that creates a new dwelling is not covered by the usual small-development exemption in the Community Infrastructure Levy guidance, so check whether your council charges it.
By the numbers
Material change
Using a single house as two or more separate dwellinghouses is a material change of use under section 55 of the Planning Act.
Source: Town and Country Planning Act 1990, section 55(3)(a): separate dwellinghouses (legislation.gov.uk)
50 m²
Minimum floor area of a one-bedroom, two-person, one-storey flat in the national space standard.
Source: Technical housing standards: nationally described space standard, Table 1 (GOV.UK)
61 m²
Minimum floor area of a two-bedroom, three-person, one-storey flat.
Source: Technical housing standards: nationally described space standard, Table 1 (GOV.UK)
2.3 m
Minimum floor-to-ceiling height, over at least 75% of the flat’s area.
Source: Technical housing standards: nationally described space standard, Table 1 (GOV.UK)
REI 30
Fire resistance that may be accepted for the structure of a converted building of up to three storeys.
Source: Approved Document B, volume 1: dwellings, paragraphs 3.18 and 6.5 to 6.7 (GOV.UK)
9 m
Greatest travel distance from a flat’s entrance door to any point in a habitable room, one fire-safety approach for a single-storey flat.
Source: Approved Document B, volume 1: dwellings, paragraphs 3.18 and 6.5 to 6.7 (GOV.UK)
100 m²
Below this size development is generally exempt from the Community Infrastructure Levy, except where it creates a new dwelling.
Source: Community Infrastructure Levy, planning practice guidance (GOV.UK)
4 storeys
A converted building of four or more storeys must meet the full standard of fire resistance in Appendix B.
Source: Approved Document B, volume 1: dwellings, paragraphs 3.18 and 6.5 to 6.7 (GOV.UK)
Two-flat conversion costs: common questions
Is the drawing fee for two flats really the biggest part of the bill?
No. The build is the biggest part. Drawings are a small share of the total, but they set the layout, so a good set can keep build cost down.
Can I convert without planning permission if the house is large?
Size does not change the rule. Splitting a house into two dwellings is a material change of use under section 55, so permission is needed whatever the floor area.
Source: Town and Country Planning Act 1990, section 55(3)(a): separate dwellinghouses (legislation.gov.uk)
Do I need separate drawings for building regulations?
Yes. Planning drawings show layout and appearance; the building regulations set adds fire separation, sound, drainage and structure. We prepare both from one survey.
Source: Approved Document B, volume 1: dwellings, paragraphs 3.18 and 6.5 to 6.7 (GOV.UK)
Sources
- Town and Country Planning Act 1990, section 55(3)(a): separate dwellinghouses (legislation.gov.uk)
- Planning fees: annual indexation from 1 April 2026, category 12 (GOV.UK)
- Technical housing standards: nationally described space standard, Table 1 (GOV.UK)
- Approved Document B, volume 1: dwellings, paragraphs 3.18 and 6.5 to 6.7 (GOV.UK)
- Community Infrastructure Levy, planning practice guidance (GOV.UK)
About the author. Ben Shaukat is Managing Director of Property Architecture Ltd, which trades as Planning Drawings.
Published by Planning Drawings. Checked against the sources above on . Planning rules change; check your council before you build.