Loft Conversions: Costs, Planning & Build Stages
Everything you need to know before starting a loft conversion in 2026 — real UK build costs by conversion type, planning permission and Permitted Development rules, building regulations, a stage-by-stage breakdown of the build, and what adds value on resale. Written by a working loft conversion design practice.
- Real 2026 UK build costs
- Cost by conversion type
- Planning vs Permitted Development
- Building regulations covered
- Stage-by-stage build process
- Resale value & ROI figures
Why homeowners choose Loft Plans
Drawings, structural calculations and planning submissions delivered as one fixed-price package — no hidden extras.
Recent project — what a finished loft looks like
Photos from a recently completed L-shape dormer in London — a worked example of what the costs and stages above actually deliver.
How it works
- 1
Survey
We visit, measure up and confirm which conversion type your roof actually supports.
- 2
Drawings
First-draft plans within 7 days, plus full building regs drawings and structural calculations.
- 3
Planning
We submit the LDC or full planning application and handle all council liaison.
How much does a loft conversion cost?
Loft conversion build costs in 2026 typically fall between £25,000 and £75,000, depending on the conversion type, your house, your specification and where in the country you are. As a guide: • Velux (rooflight-only) — from £25,000 • Standard rear dormer — £40,000–£50,000 • L-shape dormer — £55,000–£70,000 • Hip-to-gable with rear dormer — £60,000–£75,000 • Mansard or high-spec London project — £75,000–£100,000+ These figures are for the building work alone. Our drawings, structural engineer's calculations and planning submission package sit separately and are quoted as one fixed price after the survey. Expect to add 10–15% in London and the South East against the national figures above.
What drives the cost up or down?
Five factors set the build cost more than anything else. 1. Conversion type — the biggest single driver. A simple Velux is the cheapest, a mansard the dearest, with dormers in the middle. The right type depends on your roof, not your budget. 2. Floor area created — costs scale with the size of the new room. A small dormer over half the rear roof costs less than a full-width one that runs the length of the house. 3. Specification — bathroom suite quality, flooring choice, fitted joinery, glazing spec and heating system all add up. £5,000 of bathroom upgrades is easy to spend without noticing. 4. Structural complexity — if your existing floor joists are undersized or your roof has unusual framing, more steels and more strengthening are needed. Older Victorian terraces often need more structural work than 1960s semis. 5. Location — London and the South East are 10–15% higher than national averages, mostly down to labour rates and parking / scaffolding licence fees.
Cost breakdown — where does the money go?
On a typical £50,000 rear dormer conversion the spend usually splits roughly like this: • Structural work, steels and dormer framing — 25% • Insulation, plasterboard and skim — 12% • Staircase, joinery and internal doors — 10% • Electrics and lighting — 8% • Plumbing, heating extension and bathroom — 15% • Windows, roof lights and external finishes — 10% • Decoration and final fittings — 5% • Scaffolding, skips, builder's overheads and margin — 15% Drawings, structural engineer's calculations and the planning / LDC application sit outside the build cost — typically £1,500–£3,500 as a fixed price depending on conversion type.
Does a loft conversion add value to your home?
Yes — more reliably than any other home improvement. Nationwide and Halifax both put the average uplift from a well-designed loft conversion at 15–20% of property value when it adds a usable bedroom. An en-suite typically nudges that to the top of the range, and a true master suite with dressing area can do more. Worked example — a £500,000 terraced house with a new £55,000 L-shape dormer master suite: • Build cost: £55,000 • Typical value uplift: £75,000–£100,000 • Net gain on resale: £20,000–£45,000 • Plus the extra room while you live there The value comes mostly from adding a bedroom — converting an existing room to a guest bedroom adds far less than creating a new one in the loft. That's why even modest Velux conversions return well when they create a fourth bedroom.
Planning permission — when do you need it?
Most loft conversions in England don't need a full planning application. They fall under Permitted Development, which allows certain works without planning provided they meet specific limits: • Volume limits — 40 m³ added for terraces, 50 m³ for semi-detached and detached homes (the total includes any earlier extensions) • No extension beyond the front elevation • No dormer higher than the existing ridge line • Materials similar to the existing roof • Side-facing windows obscure-glazed • Roof terraces, balconies and raised platforms are not allowed under Permitted Development If you stay within those rules, you don't need planning permission — but you should still apply for a Lawful Development Certificate (LDC) from the council. It costs £135 and gives you a legal document confirming the work is lawful. Future buyers' solicitors will always ask for it. You WILL need a full planning application if your home is a flat, a listed building, in a conservation area, an Area of Outstanding Natural Beauty, or has had Permitted Development rights removed by an Article 4 direction. Front-facing dormers always need planning, even on otherwise PD-eligible houses.
Loft conversion costs by type
| Conversion type | Typical build cost | Build timeline | Planning |
|---|---|---|---|
| Velux (rooflight) | from £25,000 | 4–6 weeks | Almost always Permitted Development |
| Standard rear dormer | £40,000–£50,000 | 8–12 weeks | Usually Permitted Development |
| L-shape dormer | £55,000–£70,000 | 10–14 weeks | Often Permitted Development |
| Hip-to-gable + dormer | £60,000–£75,000 | 10–14 weeks | Usually Permitted Development |
| Mansard | £75,000–£100,000+ | 12–16 weeks | Almost always full planning |
Build costs are indicative guides only — actual figures depend on size, spec and location. Our drawings & planning package is priced separately.
Not sure which conversion suits your loft?
Book a survey — we'll measure up and tell you honestly what your roof can deliver.
Book A SurveyBuilding regulations — non-negotiable
Every loft conversion in the UK needs building regulations approval, regardless of whether planning permission is required. This is a separate process to planning and is what makes the conversion legally a habitable room rather than just a loft. Without it the conversion can't be sold as bedroom space and your home insurance may not cover it. Building regulations cover six main areas: • Structural stability — new joists, steel beams and reinforced roof framing, signed off by a chartered engineer • Fire safety — protected staircase enclosure, fire-rated doors on every habitable room on every storey below, mains-wired interlinked smoke alarms throughout • Means of escape — a clear protected route from the loft to the front door, and openable escape windows where the protected route can't be guaranteed • Thermal insulation — full insulation to current Part L energy efficiency standards in the roof, walls and floor • Stairs — minimum 2m head height (1.9m at the centre line of a fixed stair is acceptable under Part K), maximum 42° pitch • Ventilation — trickle vents on windows, mechanical extract in bathrooms and en-suites We prepare all building regulations drawings and structural calculations as part of the package. Your builder calls building control inspections at the key stages and they sign each one off in turn.
The build stages — what actually happens on site
A typical dormer loft conversion runs through eight clearly defined stages on site, usually over 8–12 weeks (longer for L-shape and hip-to-gable, shorter for Velux): Stage 1 — Set up and scaffolding (week 1). Scaffolding goes up, a skip arrives, the loft is cleared and a temporary roof cover is prepared. Stage 2 — Strip and open the roof (weeks 1–2). The relevant section of roof is stripped of tiles and felt, and the existing rafters are cut back. This is the noisiest, dustiest week — everything stays weather-tight under the temporary cover. Stage 3 — Structural work (weeks 2–4). Steel beams are dropped in by crane through the open roof and bedded into the party walls or supporting masonry. New floor joists are fixed on top of the steels — this is the new loft floor. The dormer framing is built up in timber, the new roof is felted and battened, and tiles or zinc cladding are fitted. By the end of this stage the loft is weather-tight. Stage 4 — First-fix services (weeks 4–6). Electricians run cables, plumbers run pipework for the bathroom and heating extension, insulation is fitted between rafters and joists. Stage 5 — Staircase install (week 6). The new staircase from the existing landing into the loft is fitted. The old loft hatch is removed. From this point the loft is reached by stairs, not a ladder. Stage 6 — Plasterboard and skim (weeks 7–8). Walls and ceilings are boarded and plastered. The room shape becomes obvious for the first time. Stage 7 — Second-fix and bathroom (weeks 9–11). Sockets, switches, lights, the bathroom suite, internal doors, skirting and architrave all go in. Floors are laid. Stage 8 — Snag and sign-off (week 12). Decoration is finished, the builder runs a snag list with you, and building control carry out their final inspection. On sign-off they issue the Completion Certificate — keep this with your house papers, it's what you give to the buyer's solicitor when you sell.
How long does the whole project take?
Plan for four to six months from first call to moving in. Weeks 1–3 — Survey and first draft drawings. Weeks 3–11 — Planning or Lawful Development Certificate decision (8 weeks statutory). Weeks 11–14 — Builder engagement, quote, contract, start date. Weeks 14–26 — Build on site (8–14 weeks depending on type). A Velux conversion can be quicker — 4–6 weeks on site and no front-elevation planning issue, so total project time can be three months. A full hip-to-gable or mansard in a conservation area can push past six months if planning needs amendments or neighbour objections.
What our package covers
We design and submit loft conversions across Essex, London and Hertfordshire. One fixed price covers: • Site survey and measure-up • First-draft planning drawings within 7 days • Building regulations drawings — structural, fire, insulation, ventilation • Structural calculations signed off by a chartered engineer • Lawful Development Certificate or full planning application — whichever your council requires • Full liaison with your local authority until decision • Drawings and calculations packaged for your builder and for building control We do not undertake the building work itself — that's your builder's job. We're loft conversion designers and submission specialists, and we work alongside whichever builder you choose. Most clients use a builder we've worked with before; we're happy to recommend a shortlist.
Areas we cover
We design and submit loft conversions across Essex, London and Hertfordshire — including:
Frequently asked questions
How much does a loft conversion cost in 2026?
Most UK loft conversions land between £25,000 and £75,000 all-in for the build. A Velux (rooflight-only) conversion starts around £25,000, a standard rear dormer is typically £40,000–£50,000, an L-shape dormer £55,000–£70,000, and a full hip-to-gable with dormer £60,000–£75,000. Mansard and high-spec London conversions can exceed £80,000. Drawings, structural calculations and planning fees sit on top of the build cost and we charge those as a separate fixed price.
What's included in the build cost?
Structural work (steels, joists, dormer framing), insulation to current Part L, plasterboard and skim, electrics, plumbing, central heating extension, staircase, flooring, internal doors, windows or roof lights, decoration and basic fittings. It does not normally include bathroom suites, fitted wardrobes, expensive flooring upgrades, or external scaffolding licences if your local authority charges for them.
How much value does a loft conversion add?
Nationwide and Halifax data consistently shows a well-designed loft conversion adds 15–20% to a home's value when it creates an additional bedroom — and a bedroom with en-suite often pushes that higher. On a £500,000 terrace that's £75,000–£100,000 of value added for a £45,000–£60,000 build, which is why loft conversions remain the highest-return home improvement in the UK.
Do I need planning permission?
Most loft conversions in England don't — they fall under Permitted Development if they stay within 40 m³ for terraces or 50 m³ for semis and detached homes, don't extend forward of the front roof, and don't go above the existing ridge. Conservation areas, listed buildings, flats and homes with an Article 4 direction always need a full planning application. You will still need a Lawful Development Certificate to prove the work is legal when you sell.
How long does the whole project take from first call to moving in?
Realistically, four to six months. Two to three weeks for the survey and first-draft drawings, eight weeks for the planning or LDC decision, two to four weeks to engage a builder and confirm the start date, then eight to fourteen weeks on site depending on the conversion type. Velux at the short end, full hip-to-gable at the long end.
Do I need a structural engineer?
Yes — every loft conversion needs structural calculations signed off by a chartered engineer covering new floor joists, supporting steel beams, and any changes to the roof framing. Our package includes the engineer's calculations so building control approve the structural design on first submission.
Will I need a party wall agreement?
If your home is semi-detached or terraced, almost certainly. The Party Wall Act requires you to serve notice on your adjoining neighbours before any work on a shared wall — including bedding steel beams into a party wall, which is standard on most loft conversions. A party wall surveyor handles this; budget £900–£1,500 per neighbour if they appoint their own surveyor.
Can I stay in the house during the build?
Most clients do. The first couple of weeks are noisy and dusty when the roof is opened and the steels are dropped in, but once the loft is weather-tight the noise drops sharply. Expect one or two days without water or power when the heating and electrics are extended. Bathroom-only days at the end are usually fine to live through.
Explore other conversion types
The most popular conversion type — full standing head height, mid-range cost.
Maximum-space conversion for Victorian and Edwardian terraces with a back addition.
The simplest and cheapest route — no dormer needed.
Deep dive on Permitted Development, LDCs and when you need full planning.
Ready to start your loft conversion?
Book a survey and receive your first draft drawings within 7 days.
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