Loft Conversion Costs UK — Pricing Guide for Builders and Tradespeople (2026)
A loft conversion is one of the highest-value projects a builder or main contractor can win — and one of the easiest to underquote. This guide covers full project costs across all four conversion types, a trade-by-trade cost breakdown, how builder's project management margin works on large projects, the building regs you need to know, and how to structure a quote that wins work at the right price.
Loft conversion costs in 2026 – full project figures
The figures below are complete project costs — all trades, all materials, structural engineer, scaffolding, and building regs. They are not labour-only or supply-only figures. These are what the end client pays for a finished, signed-off loft conversion in 2026.
| Conversion type | Full project cost |
|---|---|
| Velux / roof light | £20,000 – £35,000 |
| Dormer | £35,000 – £60,000 |
| Hip-to-gable | £45,000 – £70,000 |
| Mansard | £60,000 – £90,000 |
UK average 2026. London and South East will typically sit at the top of each range or above it. Costs exclude VAT where VAT applies.
If you are the main contractor managing all trades on a loft conversion, you earn a project management margin of 15–25% on top of your subcontract and material costs. This is legitimate profit for the risk, scheduling, and client management you provide — and is separate from any labour cost you put in yourself. More on structuring this in your quote below.
The four types of loft conversion explained
Which type a property can support depends on its roof pitch, ridge height, and available floor area. Getting this right before you quote — ideally with a measured survey — is the difference between a profitable job and an expensive mistake.
Velux / roof light conversion
No external structure is altered. Velux-style skylights are cut directly into the existing roof slope. This is the cheapest and fastest conversion type, and in most cases does not require planning permission (permitted development). The constraint is head height — the existing ridge must be high enough to give at least 2.2m floor-to-ceiling in the conversion after the structural floor is installed. Best suited to properties with steep roof pitches (40°+) and good ridge height. Not viable on bungalows or properties with shallow pitches.
Dormer conversion
A box-shaped extension projects vertically from the rear (occasionally side) roof slope, dramatically increasing usable floor area and headroom. A structural ridge beam is always required in a dormer — this is a critical cost item that is frequently missed on rushed quotes. Most rear dormers on houses that are not in conservation areas or listed are permitted development. The most common conversion type for terraced and semi-detached houses across the UK.
Hip-to-gable conversion
The hipped end of the roof (where the roof slopes inward to the ridge on the side elevation) is extended to create a vertical gable wall. This is most often done on end-of-terrace and detached properties where the hipped end faces a garden or side access. Often combined with a rear dormer for maximum floor area. Almost always requires planning permission. Structural complexity is higher than a dormer because of the extent of roof rebuilding involved.
Mansard conversion
The entire roof pitch is rebuilt at a near-vertical angle (72–75°), with a near-flat top section. This creates the maximum possible floor area and ceiling height but is structurally the most intensive conversion type. Mansard conversions almost always require planning permission, and many councils in conservation areas apply strict design controls. Common in London, particularly on Victorian and Edwardian terraces. Budget at the top of the range and allow significant contingency.
What drives loft conversion costs up or down
The conversion type sets the broad price band. These factors move a project toward the bottom or top of that band — and sometimes beyond it.
- →Head height and roof pitch. The minimum habitable ceiling height for a loft conversion is 2.2m. If the existing ridge height is marginal, the structural floor — which raises the floor level — may consume the available headroom. A site visit with a laser measure before quoting is non-negotiable.
- →Structural ridge beam requirement. Any dormer conversion requires a structural steel ridge beam. The size is specified by the structural engineer based on span and load. Larger beams require a crane or specialist lifting equipment — add this to your quote if access is tight.
- →Staircase position and design. The new staircase must comply with Part K (minimum 42° pitch, headroom clearances) and fire safety requirements. If the only viable staircase position takes space from an existing bedroom, this needs to be costed separately and presented to the client clearly before they sign the contract.
- →Party wall considerations. On semi-detached and terraced properties, any work within 3m of the neighbouring property's structure triggers the Party Wall etc. Act 1996. Party wall surveyor fees run to £1,000–£2,500 per neighbour. This cost is paid by the client but needs to be factored into the project programme — serving notice takes a minimum of two months before work can begin.
- →Number and size of windows. Each additional dormer window or Velux adds £600–£1,500 to materials and installation. Juliet balconies and full-height glazed doors add more. The client's specification here can move the quote by several thousand pounds.
- →Finish specification. A standard developer-grade finish and a high-spec bespoke finish on a loft conversion can differ by £15,000–£25,000 on a dormer project. Define the specification clearly in your quote or you will be managing expectation mismatches throughout the project.
Main trade costs breakdown
The figures below are what you should expect to pay subcontractors and suppliers in 2026 for a typical dormer conversion on a semi-detached house. Add your project management margin on top (see the next section).
These are subcontract and materials costs. They do not include your own labour if you are directly carrying out carpentry or other elements, nor do they include your project management margin. Building regulations fees (£400–£900 depending on local authority and application type) are typically paid by the client directly but should be noted in your quote.
Builder's project management margin
If you are the main contractor coordinating all trades on a loft conversion, you are entitled to charge a margin on all subcontract costs. This is not a markup on top of a markup — it is the legitimate return for:
- Taking contractual responsibility for the outcome with the client
- Managing the programme and sequencing all trades
- Carrying the risk if a subcontractor fails to perform or needs to be replaced
- Dealing with all client communication, variations, and sign-off
- Funding materials and subcontractor costs before client stage payments arrive
A margin of 15–25% on subcontract costs is standard in the industry. On a dormer project with £30,000 of subcontract costs, that is £4,500–£7,500 of project management margin before any direct labour you provide yourself.
How to present this without it appearing as a suspicious “markup”: do not show subcontract costs separately with a margin percentage applied. Instead, present each trade as a line item at your sell price (subcontract cost plus margin built in). The client sees a professional breakdown by trade — not a margin calculation. Include a line for “project management and coordination” as a separate fee if you want to be explicit about it, but this is optional. The key is that the total project cost is transparent and justified.
Building regulations for loft conversions
Building regulations approval is required for all loft conversions without exception. This covers structural changes, fire safety, insulation performance, and means of escape. There is no route around this, and an unconverted loft done without sign-off will cause serious problems when the property is sold.
Permitted development vs planning permission. Most Velux conversions and rear dormers on houses that are not listed or in conservation areas fall under permitted development rights — no planning application is required. Hip-to-gable conversions usually require planning permission because they alter the roof profile of the building. Mansard conversions almost always require planning permission. Always confirm with the local planning authority before starting, and advise your client to obtain a lawful development certificate for any permitted development works.
Fire safety requirements. The converted room must be accessible via a protected staircase. Every floor of the house requires a fire door (minimum FD30) to the new staircase. Mains-wired interconnected smoke alarms must be installed throughout. The ceiling below the new staircase must be 30-minute fire-rated. If a protected staircase is not achievable, an alternative means of escape (typically an escape window that meets Part B requirements, or in some cases a residential sprinkler system) must be provided. These requirements are frequently underestimated in quotes — the correct fire door and intumescent strip specification adds meaningful cost compared to a standard internal door.
Completion certificate. At the end of the project, the building control officer signs off the work and issues a completion certificate. This document is required for property sale and mortgage purposes. Make sure it is obtained before final payment is released to you, and ensure your client receives the original.
How to quote a loft conversion
A loft conversion is not a job to quote from photos or a brief phone conversation. Clients who accept a ballpark figure without a site visit are invariably the ones who dispute the final account. Clients who receive a poorly structured quote go with someone else. Here is the approach that wins work and protects your margin.
Step 1 – Feasibility estimate
After a brief site visit to assess ridge height, roof pitch, and property type, provide a written feasibility range (e.g. “a dormer on this property would typically run to £45,000–£55,000”). This sets expectation and filters out clients whose budget is not realistic. Do not charge for this visit but do confirm it in writing.
Step 2 – Measured survey and structural engineer engagement
Before producing a detailed quote on any dormer, hip-to-gable, or mansard conversion, engage a structural engineer. Their calculations determine the steel specification, joist sizing, and structural floor design. Quoting without this means you are guessing at some of your largest cost items. Some builders charge for this stage (£300–£500 to cover the engineer's initial fee); others absorb it as a cost of winning the work. Either approach is legitimate — what matters is that you do not produce a fixed-price quote without it.
Step 3 – Detailed itemised quote
Structure your quote by trade and by build stage. Include a written specification for each section (e.g. “dormer structure: treated timber frame, breather membrane, 50mm rigid insulation, cement fibre cladding, lead step flashing to existing roof”). Use provisional sums for any element that cannot be fixed-priced until a decision is made — common examples are the en-suite specification, the staircase design, and any decoration above a basic two-coat emulsion standard. State clearly what a provisional sum is and what it assumes.
Step 4 – Payment schedule
A loft conversion running for 8–12 weeks requires a milestone payment schedule. A standard structure: 10% deposit on contract signing, 25% on scaffold erect and structural floor complete, 25% on roof structure and dormer shell complete, 25% on first fix complete and plastered out, 15% on practical completion. Make this explicit in the quote — it demonstrates professional project management and protects your cash flow throughout the build.
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