A loft conversion in 2026 costs between £28,000 and £75,000 depending on the type. A basic Velux conversion starts at £28,000. A rear dormer costs £40,000–£55,000. Mansard and L-shaped conversions for maximum space cost £55,000–£75,000.
Loft Conversion Costs at a Glance
Velux (Roof Light)
No structural roof changes. Velux windows, insulation, floor, staircase, electrics and plastering. 4–5 weeks on site.
Rear Dormer
Flat-roof dormer extending from the rear. Creates a full-height room with standing space. 6–8 weeks on site.
Hip-to-Gable
Extends the sloping hip end into a vertical gable wall. Ideal for semi-detached and detached properties. 7–9 weeks.
Mansard
Entire roof slope rebuilt at 72° angle. Maximum usable space. Almost always needs planning permission. 8–12 weeks.
L-Shaped Dormer
Two dormers forming an L-shape. Common on Victorian terraces with a rear outrigger. Huge floor area. 8–10 weeks.
Hip-to-Gable + Rear Dormer
Combines both approaches for the biggest possible space on a semi-detached. Room for 2 bedrooms + bathroom.
Detailed Cost Breakdown by Type
Velux (Roof Light) Conversion — £28,000–£35,000
The simplest and cheapest option. Your builder doesn't change the roof structure at all — they fit Velux windows into the existing roof slope, strengthen the floor joists, add insulation, run electrics, plaster, and build a staircase up from below.
The trade-off is usable space. Because the roof slopes remain unchanged, you lose floor area at the edges where the ceiling drops below usable head height. You'll get a comfortable room in the centre but the edges become dead space (or low storage).
Best for: Home offices, spare bedrooms, playrooms where you don't need every square metre. Properties where the roof already has good head height (2.4m+ at the ridge).
What's included at this price: 2–3 Velux windows (£400–£800 each), floor strengthening, insulation to Building Regs standards (min 0.16 U-value), staircase, electrics (lighting, sockets, smoke alarms), plastering, and a fire door at the bottom of the new stairs.
Rear Dormer — £40,000–£55,000
The most popular loft conversion in the UK. A flat-roof box dormer is built out from the rear of your roof, creating a room with a full-height flat ceiling and vertical walls. This gives you significantly more usable floor area than a Velux.
Most rear dormers on detached and semi-detached houses fall under permitted development — no planning permission needed, provided you stay within the volume limits (40m³ terraced, 50m³ detached/semi). Dormers can't extend beyond the existing roof line at the ridge and must be set back from the eaves.
Typical spec: GRP or lead flat roof, rear window, insulated walls, new floor structure, staircase, full electrics, plumbing for a radiator, plastering, and fire-rated door.
Add an ensuite? Budget an extra £4,000–£7,000 for a compact ensuite with shower, toilet, and basin. The plumbing cost depends on proximity to existing soil stacks and water supply.
Hip-to-Gable — £50,000–£65,000
If your house has a hipped roof (slopes on all four sides rather than just two), a hip-to-gable conversion extends the sloping hip end into a vertical gable wall. This creates a much bigger room with full head height right to the end wall.
You're essentially rebuilding one end of the roof, which involves significant structural steel and temporary support. This is more complex than a dormer and costs more, but the space gain is substantial — typically an extra 4–6m² of usable floor area compared to leaving the hip.
Best for: Semi-detached houses where you extend the hip on the non-party-wall side. Detached houses can do hip-to-gable on both ends for maximum space.
Mansard Conversion — £55,000–£75,000
The most extensive (and expensive) loft conversion. The entire rear roof slope is rebuilt at a near-vertical 72° angle, with a flat roof on top. This creates a full-sized room with almost no wasted space from sloping ceilings.
Mansard conversions almost always require planning permission because they significantly alter the roofline. Budget £462 for the application in England and allow 8–12 weeks for determination. In conservation areas, mansards may be refused.
What you get: The most usable space of any conversion type. Room for a large master bedroom with ensuite and walk-in wardrobe, or two good-sized bedrooms and a bathroom. The near-vertical walls mean you can push furniture against every wall.
L-Shaped Dormer — £55,000–£70,000
Common on Victorian and Edwardian terraces with a rear outrigger (the section that extends beyond the main roof at the back). An L-shaped dormer builds two connected dormers — one on the main roof and one on the outrigger roof — creating a large L-shaped floor area.
These are complex builds. You're working with two different roof levels, and the internal layout needs careful planning to maximise the space. But the result is impressive — typically enough room for two bedrooms and a bathroom.
Regional Pricing (Rear Dormer)
Loft conversion costs vary significantly across the UK. Here's what you'll pay for a standard rear dormer conversion in each region:
What Affects the Price
Roof structure type
Traditional cut timber roofs are straightforward. Modern trussed roofs (W-shaped trusses) need to be removed and replaced with a new structure — adding £5,000–£10,000 to the job.
Staircase complexity
A straight flight from a landing is simplest (£1,500–£2,500). If you need to reconfigure the existing first-floor landing or lose a bedroom to create stair access, costs rise to £3,000–£5,000.
Ensuite bathroom
Adding an ensuite with shower, toilet and basin costs £4,000–£7,000 on top. A higher-end ensuite with walk-in shower and premium tiles pushes to £8,000–£12,000.
Structural steel requirements
Larger dormers and hip-to-gable conversions need significant steelwork. A typical dormer needs £1,500–£3,000 of steel. Complex hip-to-gable conversions can need £3,000–£6,000 of structural steel.
Fire safety upgrades
Building Regs require a protected escape route. That means fire doors on every room off the landing (£200–£350 each), fire-rated plasterboard, and interlinked smoke/heat alarms. Budget £1,500–£3,000.
Existing head height
If your ridge height is below 2.4m, a dormer or mansard is essential to get usable space — which costs more than a simple Velux. Low-pitch roofs may need the entire structure raised, adding £8,000–£15,000.
Party wall implications
If your conversion affects a shared wall with a neighbour (semi or terrace), you'll need a party wall agreement. Budget £700–£1,500 per neighbour for surveyors' fees.
Windows and glazing
Standard Velux windows cost £400–£800 each installed. Juliet balcony doors from the dormer cost £1,200–£2,500. Full-width bi-fold dormer windows are premium at £3,000–£5,000.
How to Save Money on Your Loft Conversion
💡 Smart ways to keep costs down
- Start with a Velux conversion. If your loft has good head height (2.4m+), a Velux conversion gives you a usable room for £28K–£35K — £12K–£20K less than a dormer.
- Position the bathroom above existing stacks. Putting the ensuite directly above the first-floor bathroom minimises plumbing runs and saves £1,000–£2,000.
- Use the existing staircase landing. If you can run the new stairs straight up from the existing landing without reconfiguring rooms below, you save the cost of losing and rebuilding bedroom space.
- Choose standard Velux sizes. Non-standard roof windows cost 30–50% more. Stick to common sizes like 780×980mm or 780×1400mm for best pricing.
- Do your own decoration. The builder plasters and primes. You paint, lay carpet/flooring, and fit curtains/blinds — saving £1,000–£2,500.
- Book during autumn/winter. Loft conversion specialists are busiest in spring/summer. Starting in October–February can save 5–8% and get you a quicker start date.
- Get a structural survey before committing. A £300–£500 survey from a structural engineer tells you exactly what type of conversion your roof will support — avoiding expensive surprises after work starts.
What Should Be in a Loft Conversion Quote
A comprehensive loft conversion quote should cover:
- Scaffolding — essential for dormer/mansard work (£1,500–£3,000 for a typical semi)
- Structural steelwork — beams, supports, fixings
- Dormer/roof structure — frame, roof covering, cheeks, fascias
- Floor strengthening — existing joists may need sistering or replacing
- Insulation — to Building Regs standards (U-value 0.16 or better)
- Staircase — built to Building Regs dimensions (min 2m headroom, max 42° pitch)
- Electrics — lighting, sockets, smoke alarms, consumer unit upgrade if needed
- Plumbing — radiators/heating, ensuite rough-in if applicable
- Plastering — walls and ceiling, ready for decoration
- Fire safety — fire doors, fire-rated plasterboard, interlinked alarms
- Windows — Velux or dormer windows, specified by size and type
- Waste removal — skip hire and disposal
Common exclusions: Decoration (painting/papering), flooring, bathroom fitting and sanitaryware, building regulations fees, and party wall surveyor fees. Clarify these before signing.
Hidden Costs to Budget For
Building Regulations
Full plans application. Every loft conversion needs building regs approval — this is non-negotiable.
Structural Engineer
Beam calculations, floor load checks, structural drawings. Your builder may include this or it may be separate.
Party Wall Surveyor
Per neighbour. Required if you're working on or near a shared wall. Your neighbour can appoint their own surveyor at your cost.
Planning Application
Only needed for mansard conversions or if you're in a conservation area. Most dormers are permitted development.
Boiler Upgrade
Your existing boiler may not have capacity for extra radiators/bathroom. A new combi boiler costs £2,500–£4,500 installed.
Ensuite Fitting
Sanitaryware, tiling, plumbing and fitting. Often quoted separately from the main conversion cost.
Frequently Asked Questions
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