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Conversion calculator

Turning offices, a barn, a pub or a shop into homes, and what it is worth when you do

A conversion is two questions at once. What does the work cost, and what is the building worth as something else? The gap between the two is the whole reason anybody does it, and it is usually much larger than the gap on a refurbishment because you are changing what the building is rather than what it looks like. The VAT position alone is worth fifteen points of it.

For: One use becoming another, or one building becoming several

Get it surveyed and pricedAll calculators
What you are converting
From what, to homes
Class MA permitted development, subject to prior approval and a three month vacancy test. Daylight, noise and flood risk are what prior approval actually examines, and space standards apply.
m²
Gross internal area of the building as it stands. This is what the conversion is priced on, because you are working on all of it whether it becomes saleable space or not.
%
Circulation, stairs, plant, bin and cycle stores all come out of the gross. Seventy five to eighty five per cent is normal on a flat conversion, and it is why 420 m² of building becomes 328 m² of flats.
That averages 66 m² a unit. The national space standard is 37 square metres for a one bed one person flat and 50 for a one bed two person, and it applies to permitted development conversions as well.
£
That is £2,824 a square metre of saleable area. Check it against what a comparable flat nearby actually sold for: converted units often trade slightly below purpose built ones.
£
Region
What a conversion needs that a refurbishment does not
£
New floors, openings, staircases and lift shafts. On a barn under Class Q the test is that the building is capable of conversion rather than effectively rebuilt, so if this number is large the permitted development route may not be available at all.
£
Every unit needs its own meters, its own consumer unit, its own heating and its own drainage connection. On an office with one incoming supply this is the line that surprises people, and the network operator's lead time surprises them again.
£
Parking, bin and cycle stores, private amenity space and the front door somebody can find. Usually planning conditions rather than choices.
£
Anything built before 2000 needs a refurbishment and demolition survey before work starts, and commercial buildings of that era are full of it. The survey is about £600; assuming there is none is how a programme loses two months.
Consents, fees and tax
Planning route
Faster, cheaper and much more certain where it applies. Prior approval examines daylight, noise, flooding and highways rather than the principle, and the fee is £249 per dwelling. Removed in some areas by an Article 4 direction.
%
Architect, engineer, acoustic consultant, fire engineer, SAP and building control. A conversion needs more consultants than a new build, because every one of them has to work around what is already there.
£
Charged on net additional floorspace. On a conversion of a building that has been in lawful use for six of the last three years, existing floorspace can usually be deducted, which frequently takes the charge to nothing. Worth checking properly: it is one of the largest reliefs available.
%
A conversion that changes the number of dwellings is 5%. A building empty for ten years or more, converted and then sold, can be zero rated. Everything else is 20%. Get it confirmed in writing before the contractor invoices: it cannot be fixed afterwards.
%
Fifteen per cent on a conversion, and twenty on anything listed or previously altered. You are working inside a building nobody drew accurately, and what is behind the plaster is not on any drawing.
mo
Margin on the conversion£0£925,000 of end value from 5 units, less £340,000 paid for the building and £937,074 to £1,383,366 of conversion cost. That is a margin of -89.5% to -41.2% on end value, against the 17.5% to 20% a development lender requires, and before finance. VAT at 5% rather than the standard rate is worth £165,746 of it.
£925,000End value
£937,074 to £1,383,366Conversion cost
£187,415 to £276,673Per unit
-89%–-41%Margin on end value
£165,746VAT relief worth
Building25%Conversion63%Fees and CIL7%VAT4%
What the work costs
Conversion works£588,000 to £924,000
420 m² at £1,400 to £2,200 a square metre for offices to flats, at West Midlands prices.
Structure, services and externals£105,000
The lines that make a conversion a conversion rather than a refurbishment: new floors, separate services to every unit, and the amenity space planning insists on.
Professional fees£83,045 to £116,645
10.0% of the build, plus £1,245 of planning fees and about £2,500 a unit of building control, warranty and acoustic testing.
Contingency£116,407 to £171,847
15%. On a conversion this is not padding: nobody has an accurate drawing of the building you are working inside.
VAT at 5%£44,623 to £65,875
Against the standard rate that is £165,746 saved. The contractor has to charge it correctly at the time with the evidence on file: it is not something you reclaim afterwards, and getting it wrong is not fixable later.
Total conversion cost£937,074 to £1,383,366
£2,231 to £3,294 a square metre of existing building, and £187,415 to £276,673 a unit.
What it becomes
Gross development value£925,000
5 units at £185,000, which is £2,824 a square metre of saleable area.
Saleable area created328 m²
78% of 420 m². Averaging 66 m² a unit against a 37 square metre national minimum for a one bed one person flat.
Selling costs£29,125
Agent at 2.5% plus about £1,200 a unit of conveyancing.
Total cost, all in£1,306,199 to £1,752,491
Purchase, conversion and sale. Before development finance, which on a scheme this size adds materially: price it on the development finance calculator.
Is it worth doing?
Margin on end value-89.5% to -41.2%
Below what a development lender will fund, and below the level at which the scheme can absorb anything going wrong. Either the end values are conservative, the build is over-specified, or the building is too expensive.
Uplift over doing nothing-£575,220
What the change of use creates, over and above what you paid for the building. This is the whole reason for converting rather than refurbishing.
The most you could pay for the building£0
Leaving a 20% margin on end value. Any more than that and you are buying the developer's profit and giving it to the vendor.
Cost per unit created£187,415 to £276,673
Against £185,000 of value per unit. If the cost per unit approaches two thirds of the value, there is not enough in it.
Where these figures come from

VAT. A conversion that changes the number of dwellings is charged at the 5% reduced rate, as is renovation of a dwelling empty for two years or more. New dwellings are zero rated. Everything else is standard rated. The contractor charges the correct rate at the time with the evidence on file.

Planning. Class MA permits commercial to residential conversion subject to prior approval, a three month vacancy period and a 150 square metre limit. Class Q permits agricultural to residential where the building is capable of conversion rather than requiring rebuilding. Prior approval is £249 per dwelling; a full application is £610 per dwelling below ten from 1 April 2026. Article 4 directions remove permitted development rights in some areas.

CIL is charged on net additional floorspace, and existing floorspace in lawful use for six of the last three years can usually be deducted.

Margin. Development lenders require 17.5% to 20% profit on gross development value, which is also the RICS viability benchmark.

Before development finance, which on a scheme of any size is a significant cost and has its own calculator. The VAT position is worth confirming in writing with your accountant and your contractor before anybody invoices: five per cent instead of twenty is the largest single saving on a conversion and it cannot be corrected afterwards. Permitted development rights are removed in some areas by an Article 4 direction and never applied to listed buildings, and prior approval is a real test with real refusals rather than a formality. National space standards apply to permitted development conversions as well as to full applications, so a scheme that only works at 30 square metres a flat does not work. Acoustic separation between units has to be tested to Part E, and failing the test means building it twice. An estimate, not advice and not a quotation.

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