Estimating

How to price a loft conversion

23 October 2026 12 min read

The short answer

Price a loft conversion from the structural engineer’s calculations and the Building Regulations drawings, because most of the cost sits in what they dictate: new floor joists and steels, a stair that fits the headroom, and fire protection for the escape route below. In England a habitable loft needs Building Regulations approval. Where the new storey is more than 4.5 metres above ground, Approved Document B expects the whole escape route to be protected with fire-resisting doors and partitions, and smoke alarms are needed in the circulation spaces. A dormer can often be permitted development within 40 cubic metres of added roof space on a terraced house, or 50 on others. Cutting into a party wall needs two months’ notice. Price it stage by stage, scaffold and the occupied house included, then add overhead and margin. It is standard-rated for VAT.

At a glance

Building Regulations: Approval needed for a habitable loft
Stair headroom: 2 m, or 1.9 m centre and 1.8 m side in a loft
Private stair pitch: 42 degrees maximum
Escape route doors: Fire-resisting, minimum E 20
Smoke alarms: Mains, with standby supply, in circulation spaces
Dormer volume (PD): 40 m³ terraced, 50 m³ other houses
Party structure notice: At least 2 months before work (s.3)
VAT: Standard-rated, 20%

What makes a loft conversion different to price?

A loft conversion is a new storey inside a roof that was never designed to carry one, built over a family who are usually still living underneath it. That changes three things in the price: the structure, the regulations that apply to a new storey, and access. The general method is in how to price a house extension. Before you put a figure on a loft, get:

Building Regulations drawings showing the stair, where it lands, the headroom over it and the fire doors on the escape route.
Structural calculations for the steels, their bearings, the floor joists and any roof members removed.
A survey of the roof void: height to the ridge, purlins and struts, chimney breasts, water tanks, and what the party or gable wall is built of.
The type: rooflights only, a rear dormer, or a hip-to-gable with a dormer. Each changes the scaffold, the roof work and the planning position.

The stair decides the loft. If the drawings do not show where the stair lands and how much headroom it has, the price is a guess.

What does the structure of a loft conversion involve?

The existing ceiling joists carry the ceiling below and tie the roof together. The Planning Portal says they will not have been designed to support a significant load, so a habitable loft needs a new floor, sized by the engineer, usually spanning onto new steels or load-bearing walls. Building control checks the strength of that floor and the stability of the structure, existing roof included.

Steels and bearings. Padstones in the party wall, the gable or internal walls, and how the steel gets in: through the scaffold, a window, or in bolted sections.
Floor joists. Usually set beside the old ceiling joists. Their depth comes off the headroom, so the joist size and the stair are settled together.
Roof alterations. Removing purlin struts or collar ties, framing a dormer, building up a gable. Price the engineer’s replacement design, not a guess at it.
Chimney breasts and services. Supporting a stack when a breast comes out, and moving the water tank, pipes and cables across the joists.

NoteOpening up a roof finds things: rot at the eaves, a party wall thinner than it looked. Carry a provisional sum and state its basis. See provisional sums and PC sums.

What do the stair and headroom rules say?

Approved Document K, the 2013 edition used in England, sets a private stair at a rise of 150 to 220 millimetres, a going of 220 to 300 millimetres, twice the rise plus the going between 550 and 700 millimetres, and a pitch of no more than 42 degrees. Normal headroom over the flight and landings is 2 metres.

In a loft conversion where there is not enough space for 2 metres, Approved Document K accepts 1.9 metres at the centre of the stair’s width, reducing to 1.8 metres at its side.

Alternating tread stair: only in a loft conversion with no space for an ordinary stair, serving one habitable room and, if wanted, a bathroom or WC that is not the only WC. Handrails both sides and 2 metres of clear headroom.
Fixed ladder: with fixed handrails both sides, only for a loft with one habitable room where an ordinary stair will not fit.
Retractable ladder: not to be used as a means of escape.

The stair takes space from the floor below, usually the landing or a box room, which may need a new partition and door. A made-to-measure stair is a subcontract with a lead time that cannot be measured until the new floor level is fixed. Check the headroom on site before you price it.

What fire safety does a loft conversion need?

Approved Document B volume 1 says that where a conversion creates a storey more than 4.5 metres above ground level, as a loft on a two-storey house often will, the full extent of the escape route should be addressed. Fire-resisting doors (minimum E 20) and partitions (minimum REI 30) should be provided, including upgrading existing doors where necessary, and an open-plan layout needs new partitions to enclose the escape route.

The alternative is to provide all of the following: sprinkler protection to the open-plan areas; a fire-resisting partition (REI 30) and door (E 20) separating the ground storey from the upper storeys, with the door giving loft occupants access to a first-storey escape window; and cooking facilities separated from the open-plan area by REI 30 construction.

Smoke alarms and floors

A new habitable room above the ground storey means smoke alarms in the circulation spaces: at least a Grade D2 Category LD3 system to BS 5839-6, mains-operated alarms to BS EN 14604 and a standby power supply. New floors in a loft added to a two-storey house should achieve REI 30, and the existing first floor R 30, with a relaxation only for one added storey of no more than two habitable rooms and 50 square metres.

NoteA bungalow loft that does not create a storey above 4.5 metres follows the rules for upper storeys up to 4.5 metres instead: each habitable room needs an emergency escape window or door, or direct access to a protected stairway.

For the price, that means fire doors and frames on the rooms opening onto the escape route from the ground floor up, making good around each, any new partitions, and the electrician on every storey. The loft is priced; the floors below are where the money goes missing.

What insulation standard does a loft conversion have to meet?

Turning an unheated loft into heated rooms is a change to energy status for that space under Approved Document L volume 1. New elements such as a dormer meet the limits for new fabric in an existing dwelling, and a retained roof slope worse than the threshold is upgraded.

Element	Limiting U-value W/(m²·K)
New roof, including a dormer roof	0.15
New wall, including dormer cheeks	0.18
New windows and roof windows	1.4, or Window Energy Rating Band B
New rooflights	2.2
Existing roof worse than 0.35	Upgrade to 0.16

Approved Document L volume 1, 2021 edition incorporating 2023 amendments (England). The 2026 edition, in effect from 24 March 2027, keeps these values.

Insulation costs headroom. Where meeting 0.16 would limit headroom, a lesser standard may be appropriate if the insulation and any air gap are at least the depth of the rafters and the U-value is the lowest practicable. The Planning Portal also lists sound insulation between the conversion and the rooms below among the things building control checks.

Which approvals and notices does a loft conversion need?

Permitted development for dormers and rooflights

In England a dormer falls under Class B of Part 1 of Schedule 2 to the General Permitted Development Order 2015. The headline limits:

No higher than the existing roof, and nothing beyond a front roof slope facing a highway.
No more than 40 cubic metres of added roof space on a terraced house, or 50 on any other, earlier enlargements included.
No verandah, balcony or raised platform, and no installing, altering or replacing a chimney, flue or soil and vent pipe.
Except on a hip-to-gable, the eaves kept and the dormer set back at least 0.2 metres from them so far as practicable. Side windows obscure-glazed, with no opening part lower than 1.7 metres above the floor.

Class B does not apply to flats or on article 2(3) land such as a conservation area. Rooflights fall under Class C, and may not stand more than 0.15 metres proud of the roof slope. Neither class covers a soil and vent pipe, so the pipe for a loft en suite needs checking separately. See do you need planning permission for an extension?

Building control, the party wall and CDM

A full plans application gets a decision within 5 weeks, or 2 months with consent; a building notice lets work start 2 days after submission but with no approved plans. See building control completion certificates. On a semi or terrace, cutting into the party wall for a steel, raising it, or cutting away a chimney breast from it are section 2 rights under the Party Wall etc. Act 1996, and the party structure notice goes at least two months before work begins: see party wall notices. For a homeowner, regulation 7 of CDM 2015 passes the client’s duties to you as the only contractor, and regulation 15(5) then requires a construction phase plan.

NoteWales has its own permitted development order and approved documents, and Scotland and Northern Ireland run separate systems. The Party Wall Act covers England and Wales only.

How do you build up a loft conversion price stage by stage?

Price it in the order it is built. Here is a rear dormer on a two-storey semi, one bedroom and a shower room, with every figure made up to show the method.

Stage	Labour	Materials	Plant and hire	Subcontract	Stage cost
Set-up, scaffold and preliminaries	£1,200	£300	£3,800	£0	£5,300
Opening up and strip-out	£1,100	£100	£600	£0	£1,800
Steels and new floor	£2,400	£3,300	£300	£0	£6,000
Dormer shell and roof	£4,200	£5,200	£200	£1,600	£11,200
Rooflights and windows	£500	£1,900	£0	£0	£2,400
Stair	£900	£0	£0	£3,000	£3,900
Fire doors, partitions and alarms	£1,300	£1,600	£0	£400	£3,300
First fix	£300	£300	£0	£3,100	£3,700
Insulation and plastering	£900	£2,100	£0	£2,800	£5,800
Second fix and en suite	£1,100	£900	£0	£2,900	£4,900
Finishes and making good below	£1,400	£700	£0	£0	£2,100
Direct cost	£15,300	£16,400	£4,900	£13,800	£50,400

Illustrative figures for a worked example, not market rates. Price from your own rates, the engineer’s design and your subcontractors’ quotes.

Overhead, then margin. The firm’s own labour is 450 hours at a true cost of £34 an hour; £20 of overhead on each hour adds £9,000, making £59,400, and a 12% margin on the price gives £67,500 before VAT. The method is in how to price a job properly.

Free toolMarkup vs margin calculatorEnter cost plus overhead and a target margin, and see the price before VAT it needs.

VAT goes on the whole price at 20%. HMRC’s VAT Notice 708 treats work to an existing building as standard-rated, and the reduced rate for conversions applies only where the number of single household dwellings changes.

Which costs get missed, and what should the exclusions say?

Access. Nearly everything goes up the scaffold, so its design, how long it stands and how materials are lifted are a large part of the preliminaries. A scaffold on the pavement needs a licence from the highway authority under section 169 of the Highways Act 1980, and a skip on a public road needs a skip licence. See skip and scaffold licences on the highway and work at height: the rules for builders.

The family below. Protection on the stair carpet, a sealed screen at the top of the stairs, the day the ceiling is opened, the water off while the tank moves, and a weathertight roof every night. It is all time.

Costs outside the build. The engineer, building control charges, party wall surveyors, any planning or lawful development certificate fee, and an asbestos survey on an older roof. Regulation 5 of the Control of Asbestos Regulations 2012 requires an assessment of whether asbestos is present before work that could disturb it, and HSE says buildings constructed after 2000 are unlikely to contain it. See which asbestos survey you need.

Engineer’s, building control, planning and party wall surveyors’ fees, unless included.
Rotten rafters, wall plates or brickwork found when opening up, beyond the provisional sum.
Removal of asbestos or other hazardous material.
Upgrading the boiler, hot water system or consumer unit, unless specified.
Redecorating rooms below beyond making good around the stair and alarms.
Floor coverings, blinds and furniture.

On a loft, the arguments are about the floors below: the doors, the landing and the ceilings. Put them in the scope or in the exclusions, in writing.

Whether the written figure binds you depends on its wording: see is a builder’s quote legally binding?

How does Estimark help you price a loft conversion?

Estimark builds the estimate from your own price book: your own items, favourites for the lines on every job, and assemblies for groups you price as one, such as a fire door with its frame. Labour rates per trade, overhead recovery and margin belong to the company, and an old estimate keeps the rates it was built on. Materials, labour, plant and subcontract stay separate. See tenders and estimating.

The estimate becomes the quote, with your terms and a valid-until date, which the client opens from a link and accepts and signs online without an account. The accepted quote converts into a job carrying its total as the budget. See quotes and online acceptance.

Sources

Questions

Frequently asked

Does a loft conversion need planning permission?

Often not, in England. A dormer is permitted development under Class B if it adds no more than 40 cubic metres of roof space on a terraced house or 50 on others, rises no higher than the existing roof, does not project from a front slope facing a highway and meets the other conditions. Flats and conservation areas are different.

Does a loft conversion need Building Regulations approval?

Yes, to make it a habitable room. Building control checks the new floor, the roof’s stability, the stair, the escape route and sound insulation, and the homeowner will need the completion certificate when they sell.

What headroom does a loft conversion stair need?

Approved Document K asks for 2 metres. Where a loft conversion has not enough space, it accepts 1.9 metres at the centre of the stair’s width, reducing to 1.8 metres at the side.

Do I need fire doors for a loft conversion?

Usually. Where the loft creates a storey more than 4.5 metres above ground, Approved Document B expects fire-resisting doors (minimum E 20) and partitions (minimum REI 30) along the escape route, upgrading existing doors where necessary, unless the sprinkler alternative is followed.

Is VAT charged on a loft conversion?

Yes, at 20% where the builder is VAT-registered. HMRC treats work to an existing building as standard-rated, and the reduced rate for conversions applies only where the number of dwellings changes.

Does a loft conversion need a party wall notice?

Often, on a semi or a terrace. Cutting into the party wall for a steel, raising it, or cutting away a chimney breast from it is section 2 work, and the notice goes at least two months before it begins.

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