For owners of large London houses
Cost to Convert a House into Flats
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 4 min read
The short answer
The cost of converting a house into flats is set by the building, not the flat count. Structure, fire and sound separation and new services usually outweigh the finishes.
- New stairs, openings and steel beams are the first big cost in a period house.
- Every flat needs fire and sound separation, and sound tests before completion.
- Separate meters and supplies for each flat mean work with the network operators.
- Planning, CIL and party wall costs come before a single wall moves.
The cost to convert a house into flats is decided by the building, not by the number of flats on the drawing. A tall period house with sound floors and a good stair is one budget. The same house with weak joists, one narrow stair and a single gas meter is another.
This guide sets out what drives a house to flats conversion cost in London, in the order it lands on the budget. It does not quote a price per flat, because a figure without a survey is a guess.
We design and build the work of converting a house into flats for owners of large London houses. Own a house that could become flats? Book a site visit and we will price it from a real scope.
What drives the house to flats conversion cost
Five things set the budget. The first three are the build; the last two decide whether it is worth doing at all. Ask any builder how their quote covers each line. A quote that prices only the finishes has left out the expensive part. Each driver is explained below, in the order it lands on the budget.
| Driver | What it covers | Why it moves the cost |
|---|---|---|
| Structure and stairs | New openings, steel beams, a protected common stair | Period houses rarely have the stair or the floors a flat block needs |
| Fire and sound separation | Floors and walls between flats, fire doors, an escape route | Every flat must be separated, then tested |
| Services and meters | Supplies, meters, drainage and heating for each flat | One house becomes several metered homes |
| Planning, CIL and party walls | Application, levies, neighbours' notices | Costs and time before work starts |
| Value against cost | The finished flats against the total spend | Decides if the scheme stands up |
Structure and new stairs
A house has one family stair and floors built for one household. Flats need a protected route out of every home and floors that carry new kitchens and bathrooms. That is where a conversion budget starts.
In practice it means new openings and steel beams, strengthened or replaced joists, and often a reworked stair enclosed as a common escape route. In a large period house the stair is usually worth keeping, but the walls and doors around it change.
An engineer's survey before design tells you which of these you face. It is the cheapest money in the project.
Fire and sound separation
Each flat must be separated from the others for fire and sound. It is easy to underestimate, because it sits inside the floors and walls.
Old timber floors rarely meet the standard as they are. New ceilings, insulation and resilient layers go in, and they take ceiling height with them. Fire doors, a protected stair and alarms complete the fire strategy.
Services, meters and utilities
One house becomes several homes, and each needs its own supplies. Usually that is a separate electricity supply and meter per flat, and often separate water metering and heating.
New supplies are arranged with the electricity and water network operators. Their lead times sit outside your builder's control, so they belong in the programme from the start. Drainage follows the same logic: every new bathroom and kitchen needs a route to a soil stack.
Many conversions now give each flat its own electric heating and hot water. That avoids running new gas supplies to every flat, but it changes the electrical load the building needs.
Hired Craftex for our conversion project. We had to convert a 600m2 office into 8 apartments. Guys did an excellent job in terms of program efficiency and quality of the finishes which gave us a big upside to exit the project 2 months faster than other competitors’ timelines.
Planning, CIL and party walls
Splitting a house into flats is a change of use, so it needs planning permission. The council charges a fee for each additional home; our guide to planning application fees has the current rates. Drawings, surveys and any reports the borough asks for come on top.
The Community Infrastructure Levy is charged on new floorspace. Floorspace that has been in lawful use can usually be deducted, so a split inside the existing walls may owe little. A new storey, a basement or an extension adds liability.
Work to a shared wall needs a party structure notice at least two months before it starts. Allow for the surveyors' fees as well.
Value of the flats against the cost
A conversion works when the finished flats are worth more than everything spent. That includes the house, the build, fees, finance, levies and a contingency for what the strip-out uncovers.
The space standard decides how many flats you can sell. A two-bed, four-person flat needs 70 m² on one level, so a house that falls just short loses a bedspace or a flat. Check the layout against the space standards for flats before you trust any appraisal.
How we price a conversion
We price from the building itself. A site visit and a structural and services survey come first, then a scope for each flat and the common parts. Our house to flats conversion calculator gives a first figure for your floor area before that.
Construction is an in-house department, and around 60% of the labour is our own. So the people who price the separation details are the people who build them.
Already holding a quote? Send it to us. We'll beat it for the same scope, and we reply the same working day. Or book a site visit and we will walk the house with you.
