Loft Conversion Cost Calculator UK (2026)

How to use the loft conversion calculator

The calculator asks seven questions. Each one changes the estimate, so it is worth a few minutes getting them right – especially the floor area, which drives most of the total.

1. Loft conversion type

Choose the type that matches how much the roof needs to change:

• Velux / rooflight – windows added to the existing roof slope, roof shape unchanged. Only works if you already have good headroom.
• Dormer – a box structure built out from the rear slope to create vertical walls and full-height space. The UK’s most common type; see our dormer loft conversion cost guide.
• Hip to gable – the sloping side of a semi or detached house is rebuilt as a vertical gable wall, usually combined with a rear dormer. See the hip to gable cost guide.
• Mansard – one or both slopes are rebuilt almost vertically with a flat top, giving the most space. See the mansard cost guide.

If you are not sure, start with dormer, then compare the result with other types.

2. Floor area (m²)

Enter the usable floor area of the finished room: the part with at least 1.5m of headroom once the conversion is built. This is the area loft companies and our per-m² rates are based on.

• Check the headroom first. Measure from the top of a ceiling joist to the underside of the ridge timber. Around 2.2m or more is generally the minimum for a practical conversion without raising the roof.
• Velux conversions: measure the floor zone where you can stand at 1.5m or more today, and multiply its length by its width.
• Dormers and mansards: the new walls create more full-height space, so the usable area is larger than what you have now. As a rough guide, take the length of the dormer and multiply by the depth from the dormer face to where the front slope drops below 1.5m. An architect’s drawings will give the exact figure.

Typical sizes: a single bedroom or office is about 20m², a double bedroom 25m², and a bedroom with an en-suite 28–35m². Bungalows can be larger – see our bungalow loft conversion guide.

3. Finish level

• Basic – simple layout, standard fittings, no bathroom.
• Standard – mid-range finishes and a shower room or small en-suite.
• High-end – bespoke joinery, larger glazing and a quality en-suite.

4. Region

Pick the region where the house is. Labour, access and demand push London prices well above the North of England and Wales.

5. Party wall agreements

If you live in a terrace or semi and the work affects the shared wall (steel beams are usually bearing on it), you must serve party wall notices. If neighbours consent in writing, choose “not needed”. If they dissent, select one or two neighbours and we add a surveyor for each. Our party wall agreement guide explains the process.

6. Planning application

Most rear dormers, Velux and many hip to gable conversions are permitted development, so leave this unticked and we add £274 for a lawful development certificate. Tick it for a mansard, a front dormer, a flat or maisonette, or a home in a conservation area or under an Article 4 direction. Our loft conversion planning permission guide explains the 40m³ and 50m³ volume limits.

7. VAT

Leave VAT on unless your builder is not VAT-registered. Most loft conversion companies charge 20%.

How we calculate your loft conversion cost

The calculator multiplies your floor area by a build cost per m² for the type and finish you choose, adjusts it for your region, then adds professional fees, the planning or certificate fee, any party wall costs and VAT. Totals are rounded to the nearest £500.

Build cost per m² by loft type

Loft conversion type	Basic (£/m²)	Standard (£/m²)	High-end (£/m²)
Velux / rooflight	610 – 830	700 – 960	870 – 1,170
Dormer	1,090 – 1,520	1,220 – 1,740	1,520 – 2,090
Hip to gable	1,350 – 1,780	1,480 – 2,000	1,830 – 2,430
Mansard	1,480 – 2,090	1,650 – 2,350	2,090 – 2,870

Loft conversion cost per m² by region

We apply a regional multiplier to the build cost. This table shows the resulting standard-finish rates.

Region	Multiplier	Velux (£/m²)	Dormer (£/m²)	Hip to gable (£/m²)	Mansard (£/m²)
London	1.25	875 – 1,200	1,525 – 2,175	1,850 – 2,500	2,062 – 2,938
South East	1.12	784 – 1,075	1,366 – 1,949	1,658 – 2,240	1,848 – 2,632
South West & East of England	1.04	728 – 998	1,269 – 1,810	1,539 – 2,080	1,716 – 2,444
Midlands	0.98	686 – 941	1,196 – 1,705	1,450 – 1,960	1,617 – 2,303
Scotland & Northern Ireland	0.96	672 – 922	1,171 – 1,670	1,421 – 1,920	1,584 – 2,256
North of England & Wales	0.92	644 – 883	1,122 – 1,601	1,362 – 1,840	1,518 – 2,162

Fees, VAT and contingency

Item	What we add
Architectural design and drawings	4–7% of the build cost
Structural engineer	£500 – £2,500
Building control	£600 – £1,200
Lawful development certificate (default)	£274
Householder planning application (if ticked)	£548 instead of £274
Party wall surveyor	£1,000 – £3,000 per neighbour
VAT	20% on everything except the planning or certificate fee
Recommended budget	Total plus 10% contingency

Example results

Here is what the calculator returns for some common projects. All include design, structural engineer, building control, the certificate or planning fee and VAT.

Project	Region	Estimated total	With 10% contingency
25m² Velux, standard	Midlands	£23,000 – £35,000	£25,500 – £38,500
25m² Velux, standard	London	£29,000 – £43,000	£32,000 – £47,500
25m² dormer, standard	Midlands	£39,000 – £59,500	£43,000 – £65,500
30m² dormer, standard	Midlands	£46,500 – £70,500	£51,000 – £77,500
30m² dormer, standard	North of England & Wales	£43,500 – £66,500	£48,000 – £73,000
30m² dormer, standard	London	£58,500 – £88,500	£64,500 – £97,500
30m² dormer, standard, planning application and one party wall surveyor	Midlands	£48,000 – £74,500	£52,500 – £81,500
30m² hip to gable, standard	Midlands	£56,000 – £80,000	£61,500 – £88,000
30m² hip to gable, standard	London	£71,000 – £101,000	£78,000 – £111,000
35m² mansard, standard	Midlands	£72,000 – £108,000	£79,500 – £119,000

These sit close to published averages. The HomeOwners Alliance, citing Checkatrade, puts the average UK loft conversion at about £50,000 in 2026, and MyBuilder gives £40,000–£60,000 for a typical dormer. For a full comparison of types and house styles, see our loft conversion cost guide, or the London loft conversion cost guide if you live in the capital.

What the calculator includes – and what it doesn’t

Included in the build cost per m²

• Structural work: steel beams, new floor joists and roof alterations for the chosen type.
• Dormer, gable or mansard shell, roofing, insulation and windows or rooflights.
• A new staircase of standard design.
• Fire safety work: fire doors on the escape route and mains-powered interlinked smoke alarms.
• Electrics, heating, plastering and decoration.
• A shower room or en-suite at the chosen specification (standard and high-end only).
• Standard scaffolding for the build.

Not included

• Furniture, curtains, blinds and freestanding storage.
• Unusual structural work, such as raising the roof ridge, underpinning or major repairs to rotten timbers.
• Asbestos removal, or moving a boiler or cold water tank.
• Pavement scaffold licences, hoarding and parking suspensions, which councils charge for separately and which are common in London.
• Any repairs to neighbouring property under a party wall award.
• The 10% contingency, which is shown separately as a recommended budget.

How accurate is the loft conversion price calculator?

The calculator is designed to give a realistic budget range, not a quote. The low end assumes a straightforward loft with a traditional cut roof, easy access and an efficient builder; the high end allows for a more complex structure, a busier market or a higher-spec finish within that level. Most quotes for typical projects should fall within the range.

Your price could fall outside it if:

• Your roof uses trussed rafters (common in post-1960s homes), which need more steelwork.
• Headroom is tight and the ceiling below has to be lowered or the roof raised.
• The staircase cannot fit above the existing one without reworking the floor below.
• Materials have to be carried through the house, or access is restricted.
• You want large glazed gables, Juliet balconies or bespoke joinery beyond the high-end level.

Use the estimate to set a budget and to sense-check quotes. Then get at least three detailed, itemised quotes from loft specialists who have surveyed your roof. Our methodology page explains our data sources in more detail. Planning how to pay for it? See how to finance an extension or loft conversion.

Loft conversion calculator FAQs

How much will my loft conversion cost?

It depends mainly on type and size. Using our calculator for the Midlands with a standard finish, a 25m² Velux conversion costs about £23,000–£35,000, a 30m² dormer £46,500–£70,500, a 30m² hip to gable £56,000–£80,000 and a 35m² mansard £72,000–£108,000, including fees and VAT.

What is the cost per m² of a loft conversion?

Build costs for a standard finish range from about £700–£960 per m² for a Velux conversion to £1,650–£2,350 per m² for a mansard, before VAT and fees. The HomeOwners Alliance gives £1,250–£2,500 per m² as a typical range across types. Smaller lofts cost more per m² because the staircase, steels and fees are fixed costs.

Does the calculator include VAT?

Yes, by default. VAT at 20% is added to the building work, fees and party wall costs, but not to the planning or certificate fee. Untick the VAT box if your builder is not VAT-registered.

How do I measure my loft for a conversion?

First measure from the top of a ceiling joist to the underside of the ridge – about 2.2m or more is generally needed. Then estimate the floor area that will have at least 1.5m of headroom after the conversion. For dormers and mansards this is larger than the space you have today.

Can I get a loft conversion for £30,000?

Possibly, if it is a Velux conversion of around 25m² outside London, which our calculator puts at £23,000–£35,000 in the Midlands. A dormer of a useful size usually costs at least £39,000 once fees and VAT are included.

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