Loft Conversion Cost Calculator UK | Dormer, Velux

How to use the loft conversion cost calculator

Pick the Type of loft conversion: rooflight (Velux), dormer, hip-to-gable or mansard.
Enter the New floor area, the usable floor area of the new room or rooms. A builder or architect can confirm how much your loft will give.
Switch on Add an en-suite shower room if you want one priced in.
Choose Where do you live? to adjust for your area.
Check the range, then tap Add to shopping list to keep it, or Share link to send it to your builder.

Why use this calculator

It prices by loft type, so you can see how much more a dormer, hip-to-gable or mansard costs than a simple rooflight conversion.

It scales with the size of your loft, and it checks itself. It compares your answer with published whole-loft prices for a 30 m² conversion.

It's clear about what's left out. The result is a build cost, so architect, engineer, building control and party wall fees are not in it.

It shows its working. Under How we worked it out you can see every step with your own numbers.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

A dormer conversion costs £1,150 to £2,000 per m² (typical £1,600) for a bedroom and stairs.
Build cost = 30 m² × £1,150 to £2,000 = £34,500 to £60,000.
Prices are for the UK average, so no regional adjustment.
Add it up: low £34,500, typical £48,000, high £60,000.
Sense check: published prices for a whole dormer loft conversion of about 30 m² are £40,000 to £60,000 (typical £50,000). The same size from our rates is £34,500 to £60,000.

How we estimate the cost

We multiply your new floor area by a price per m² for the type of conversion. These rates are for a bedroom and stairs:

Type	Per m² (low to high)	Typical
Rooflight (Velux)	£700 to £1,100	£900
Dormer	£1,150 to £2,000	£1,600
Hip-to-gable	£1,400 to £2,330	£1,900
Mansard	£1,500 to £2,670	£2,150

If you add an en-suite, we use a small bathroom fit-out as a stand-in: £2,200 to £3,500 to £5,500 (low, typical, high). That figure is for a small bathroom from one source, not a loft en-suite. Your region multiplier applies to every line, including the en-suite.

At the end we compare the answer with published prices for a whole loft conversion of about 30 m²: rooflight £22,500 to £30,000, dormer £40,000 to £60,000, hip-to-gable £50,000 to £70,000 and mansard £50,000 to £80,000.

What’s included

The build cost of a new floor, stairs, roof work, insulation and a finished bedroom, at the per-m² rate.
An en-suite shower room, if you switch it on.
Not included: architect, structural engineer, building control and party wall fees, and VAT if a quote adds it.

What changes the price

The type. A dormer, hip-to-gable or mansard costs more than a rooflight conversion because more of the roof is rebuilt.
The size. Stairs and structure are a fixed cost, so small conversions cost more per m². We warn you below 15 m² and above 45 m².
The en-suite. Extra plumbing and waste routing in a loft can add to it.
Your area. We use London ×1.25, South East ×1.15, Scotland ×0.93, Wales ×0.92, the North ×0.92 and Northern Ireland ×0.88. Loft guides suggest London can be higher still.

Tips

Get at least three quotes, and ask each to price the same drawings. Ask what's included for stairs, insulation, electrics, heating and decorating.
Measure your loft. Headroom and floor area both matter, and a structural check will tell you what's possible. Ask a builder or architect.
The type of conversion changes the price a lot. A rooflight conversion keeps the roof shape. A mansard rebuilds the whole slope.
Plan for fees on top. Architect, structural engineer, building control and party wall costs are not in our result. See our house extension cost calculator for typical fee ranges.
Not sure about planning? The planning permission checker gives you the national rules for houses in England. Your council makes the decision.
Got a quote already? Put it through the quote checker to see how it compares.

FAQs

How much does a loft conversion cost in the UK?

For a 30 m² loft, about £21,000 to £33,000 for a rooflight conversion, £34,500 to £60,000 for a dormer, £42,000 to £69,900 for a hip-to-gable and £45,000 to £80,100 for a mansard. Published whole-loft prices are in a similar range, but fees are extra.

How much does a loft conversion cost per square metre?

About £700 to £1,100 per m² for a rooflight conversion, £1,150 to £2,000 for a dormer, £1,400 to £2,330 for a hip-to-gable and £1,500 to £2,670 for a mansard. These cover a bedroom and stairs. Small lofts cost more per m².

How much does a dormer loft conversion cost?

About £34,500 to £60,000 for 30 m², with a typical price of £48,000. That is £1,150 to £2,000 per m² (typical £1,600). Published prices for a whole dormer conversion are about £40,000 to £60,000.

How much does a Velux (rooflight) loft conversion cost?

About £21,000 to £33,000 for 30 m², typically £27,000. That is £700 to £1,100 per m² (typical £900). It is the cheapest type because the roof shape stays the same.

How much does an en-suite in a loft conversion cost?

We add about £2,200 to £5,500 (typical £3,500), based on a small bathroom fit-out from one source. A loft en-suite can cost more because of the extra plumbing and waste routing, so ask for it as a separate line.

Do I need planning permission or Building Regulations approval for a loft conversion?

Building regulations approval is normally needed, and some conversions need planning permission too, depending on the type and the changes to the roof. Ask your council before you start.

Is VAT charged on a loft conversion?

Check each quote, because our guide prices don't all say. Ask your builder to state whether VAT is in the price.