Loft Conversion London 2026: Costs, Types & Planning Guide
A loft conversion adds 20–35m² of living space without touching your garden. Here are the real 2026 costs, which types suit which properties, and what London's planning rules mean for your project.
Quick Answer
Loft conversions cost £50k–£120k+ inc VAT in London depending on type. Velux £50k–£65k, Dormer £60k–£80k, Hip-to-gable £65k–£95k, Mansard £80k–£120k. Most rear dormers qualify for permitted development. All loft conversions need building regulations regardless.
£50k–£65k
Velux (simplest)
£60k–£80k
Dormer (most popular)
15–25%
Value added
Check your specific property constraints
Free Property CheckIs My Loft Suitable?
Before anything else, check these four things. They determine whether your loft is convertible and what type is realistic.
1. Head height: minimum 2.2m at the ridge
Measure from the top of the ceiling joists (not the finished ceiling below) to the underside of the ridge board. Less than 2.2m and your options are limited to very small Velux conversions or raising the roof (expensive and always needs planning).
2. Roof type: pitched or flat?
Most London terraces have a pitched gable or hip roof. Flat-roofed additions cannot be converted without major structural work. Hip roofs are candidates for hip-to-gable conversion.
3. Floor joist size and span
Existing ceiling joists are designed for light storage, not habitable loads. Most need sistering (doubling up) or full replacement with deeper joists. A structural engineer will specify this once you have outline drawings.
4. Staircase location
A compliant staircase needs roughly 3m length and 800mm minimum width. Work out where it will go before committing to a design. Losing a bedroom on the floor below to accommodate stairs changes the project economics entirely.
Types of Loft Conversion: Costs & What They Suit
Velux (Rooflight) Conversion
The most affordable option. Windows are installed flush into the existing roof slope. The structure is unchanged, which keeps costs down and almost always qualifies for permitted development.
- Almost always permitted development (no planning needed)
- Fastest build: 4–6 weeks on site
- Least disruptive to neighbours
- Requires at least 2.4m at ridge for a usable room
- Less usable floor area than a dormer
Best for: Properties with good existing head height wanting a simple bedroom or office
Rear Dormer Conversion
A box extension that projects out from the rear roof slope, creating full-height walls at the back. The most popular loft conversion in London. Adds significant usable floor area and headroom.
- Rear dormer typically qualifies for permitted development
- Works on most London terraced and semi-detached houses
- Enough space for a bedroom and en-suite
- Party wall notice needed for terraced/semi-detached properties
- Steel ridge beam required (included in cost)
Best for: Most London terraced and semi-detached houses
Hip-to-Gable Conversion
Converts a sloping hip end to a vertical gable wall, extending the ridge line and creating a large, full-height room. Only possible on semi-detached or detached houses with hipped roofs. Usually combined with a rear dormer.
- Substantially increases usable floor area vs dormer alone
- Can accommodate two rooms plus bathroom
- Permitted development on most semis but check Article 4 directions
- Only works on semi-detached or detached properties with hip roofs
Best for: Semi-detached or detached houses in outer London (Wimbledon, Ealing, Barnet)
Mansard Conversion
Rebuilds the rear roof slope as a near-vertical wall with a flat or shallow-pitched top. Creates the most floor area and highest ceilings of any loft type. The most common approach in central and inner London. Always requires planning permission.
- Maximum floor area (often 50%+ more than a dormer)
- Full-height ceilings throughout
- Can create two bedrooms plus bathroom at loft level
- Always needs planning permission (8–12 weeks, £528 fee)
- Party wall agreements required on both sides in a terrace
Best for: Central and inner London terraces (Hackney, Islington, Lambeth, Southwark)
2026 Cost Summary
All costs below include VAT at 20%. All-in figures add architect fees, structural engineer, building regulations, and a 10% contingency.
| Type | Construction | All-in | On site |
|---|---|---|---|
| Velux | £50k–£65k | £60k–£78k | 4–6 weeks |
| Rear dormer | £60k–£80k | £72k–£98k | 6–10 weeks |
| Hip-to-gable | £65k–£90k | £78k–£110k | 8–12 weeks |
| Hip-to-gable + dormer | £80k–£115k | £96k–£138k | 10–14 weeks |
| Mansard | £80k–£120k | £95k–£145k | 10–14 weeks |
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Planning Permission vs Permitted Development
Most loft conversions in London fall under permitted development (Class B), which means no planning application. But there are firm limits, and London's conservation areas and Article 4 directions remove these rights from many properties.
Permitted development rules for loft conversions
All of these must apply:
- Volume added does not exceed 40m³ for terraced houses or 50m³ for semi-detached and detached
- No part of the extension protrudes beyond the existing roof plane facing a highway
- The dormer sits at least 20cm back from the eaves and 20cm from party walls
- Materials are similar in appearance to the existing house
- No balconies, verandas, or raised platforms
- The roof does not exceed the highest part of the existing roof
Conservation areas and Article 4 directions
In a designated conservation area, permitted development rights for loft conversions are removed entirely. You need planning permission for any dormer or roof extension. Many inner London boroughs (Islington, Hackney, Southwark, Camden) have significant conservation area coverage.
Article 4 directions work similarly. Check your borough's planning portal or use our property tool to confirm your property's status before instructing an architect.
| Type | Standard house | Conservation area |
|---|---|---|
| Velux (rear) | Permitted development | Planning permission required |
| Rear dormer | Usually PD (volume limits) | Planning permission required |
| Hip-to-gable | Often PD on semi-detached | Planning permission required |
| Front/side dormer | Always planning permission | Planning permission required |
| Mansard | Always planning permission | Planning permission required |
Not sure if your loft needs planning permission? Our AI checks your property, including conservation area and Article 4 status.
Check now — it's free →Building Regulations: What Every Loft Must Meet
Planning permission (or the absence of it) has no bearing on building regulations. Every loft conversion needs building regulations approval. Building control inspects during construction and issues a completion certificate at the end. Without it, you cannot sell the property.
Head height
- 2.2m minimum at the highest point (2.3m+ recommended for resale)
- 2.0m minimum over the staircase and landings
- At least 50% of the floor area must have full standing height to count as habitable space
Structural requirements
- Floor joists sized for habitable loads (1.5 kN/m² minimum live load)
- Steel ridge beam or similar for dormer/mansard roof structure
- Load path from new structure down to foundations (structural engineer to confirm)
- Party walls: gable wall may need raising and tying in to new structure
Fire safety
- 30-minute fire-resisting ceiling between loft floor and floor below (REI 30)
- Fire doors (FD30S rated) on all rooms leading onto the escape route
- Protected stairwell enclosure with fire-resistant construction
- Mains-powered, interlinked smoke alarms on every floor including the new loft
- Escape window where the floor level exceeds 4.5m above ground (if no protected stair)
Insulation (2026 standards)
- Roof: U-value 0.16 W/m²K or better (requires 250–300mm total insulation depth)
- Windows: U-value 1.4 W/m²K or better (standard modern double or triple glazing)
- Adequate ventilation to prevent interstitial condensation in the roof buildup
Staircase
- Minimum 800mm clear width (900mm is better)
- Maximum pitch: 42 degrees
- Consistent rise (max 220mm) and going (min 220mm)
- Handrails both sides if width exceeds 1m
- Headroom of 2.0m over the full stair length
Party Wall: What Triggers It and What It Costs
Most loft conversions in terraced or semi-detached houses trigger the Party Wall Act 1996. This is separate from planning and building regulations.
Work that triggers a party wall notice:
- Any work to the party wall itself (raising, cutting into, tying new structure in)
- Steel beams bearing into or through the party wall (common in dormers and mansards)
- Raising the party wall (common in mansard conversions)
The party wall timeline
- 1Serve notice on neighbours at least 2 months before work starts
- 2Neighbours have 14 days to consent in writing or dissent
- 3No response counts as dissent. Appoint a party wall surveyor (or agree a single agreed surveyor)
- 4Surveyor prepares the Party Wall Award (a legally binding document). Takes 4–8 weeks after appointment
- 5Work can start once the Award is signed by both surveyors
Cost: You pay for both your surveyor and your neighbour's. Budget £800–£1,400 per neighbour. In a terrace with neighbours on both sides that can be £1,600–£2,800.
Tip: Start the party wall process in parallel with planning and building regulations. It runs on its own track but adds real time if you leave it too late.
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Realistic Project Timeline
Design and drawings
3–5 weeksArchitect visits, survey, planning drawings, structural calculations
Building regulations submission
3–5 weeksFull plans application to local authority building control
Planning permission (if needed)
8–12 weeksRequired for mansard, front/side dormers, conservation areas. Runs in parallel with building regs
Party wall process (if triggered)
2–4 monthsNotice period plus Award preparation. Runs in parallel but must start early
Construction on site
4–14 weeksVelux: 4–6 weeks. Dormer: 6–10 weeks. Hip-to-gable or mansard: 10–14 weeks
Total project
4–8 monthsPermitted development with no party wall issues: 4–5 months. Mansard with planning and party wall: 7–9 months
Common Mistakes
Not measuring head height before commissioning design
Measure from the top of the ceiling joists to the underside of the ridge. If you measure from the finished ceiling you will overestimate by 100–150mm. Many projects stall because the loft is too shallow once this is correctly measured.
Underestimating staircase impact on the floor below
A compliant staircase typically takes 3.0–3.5m of floor length and 1.0m of width. If the only viable location is through an existing bedroom, you may lose a room on the floor below. Factor this into the net value calculation before proceeding.
Assuming PD without checking conservation area or Article 4
Conservation areas are widespread in inner London. Many homeowners commission architect drawings before checking, then find they need full planning permission and the design needs changing. Take 10 minutes to check your borough's planning portal first.
Starting the party wall process too late
You must serve notice at least 2 months before work starts on any party wall. If a neighbour dissents, add another 4–8 weeks for the Award to be agreed. Serve notice the moment you have an outline scheme, not once you have building regulations approval.
Not accounting for insulation depth in headroom calculations
To meet the 0.16 W/m²K requirement you need 250–300mm of insulation in the roof buildup. Whether installed between and below rafters or above the deck, this eats into the apparent headroom. An architect will show this in section drawings, but check the finished floor-to-ceiling dimension early.
Using a builder without building regulations sign-off
Some builders offer to do loft conversions without building regulations, cutting upfront cost. This creates problems when you sell (solicitors will ask for the completion certificate), can invalidate home insurance, and leaves structural and fire safety work unchecked. Always use a builder who works through building regulations.
Frequently Asked Questions
How much does a loft conversion cost in London in 2026?
Costs in 2026 inc VAT: Velux £50k–£65k, Rear dormer £60k–£80k, Hip-to-gable £65k–£90k, Hip-to-gable with rear dormer £80k–£115k, Mansard £80k–£120k. All-in costs including architect, structural engineer, building regulations, and contingency run 15–20% higher. Add £8k–£15k for an en-suite bathroom.
Do I need planning permission for a loft conversion?
Most rear dormers and Velux conversions qualify for permitted development (no planning application). You need full planning permission for: mansard conversions, front or side dormers, properties in conservation areas, and properties subject to Article 4 directions. Planning takes 8–12 weeks and costs £528 in application fees.
What is the minimum head height for a loft conversion?
Building regulations require 2.2m minimum at the highest point and 2.0m over the staircase. Measure from the top of the ceiling joists (not the finished ceiling below) to the underside of the ridge board. Allow for 250–300mm insulation depth within the roof structure. 2.3m+ is recommended for comfort and resale value.
Do I need a party wall agreement for a loft conversion?
Yes, if you are in a terraced or semi-detached property and the work touches the party wall (raising it, cutting into it, or bearing steelwork through it). Serve notice at least 2 months before work starts. If neighbours do not consent in writing, appoint a party wall surveyor. Budget £800–£1,400 per neighbour for surveyor fees.
How long does a loft conversion take?
On-site construction: Velux 4–6 weeks, Dormer 6–10 weeks, Hip-to-gable or Mansard 10–14 weeks. Total project from design to completion: 4–5 months for a straightforward PD dormer, 7–9 months for a mansard with planning permission and party wall process.
Which type of loft conversion suits a London terraced house?
A rear dormer is the most common choice for terraced houses. It usually qualifies for permitted development, fits within the 40m³ volume allowance, and creates a usable bedroom with en-suite. In inner London conservation areas, a mansard may be required or preferred by planners, but it costs more and always needs planning permission.
What building regulations apply to a loft conversion?
All loft conversions need building regulations approval regardless of whether planning permission is required. Key requirements: structural floor joists sized for habitable loads, fire-resisting ceiling (REI 30), FD30S fire doors on the escape route, mains-wired interlinked smoke alarms on every floor, compliant staircase (42 degree max pitch, 800mm min width), and roof insulation to U-value 0.16 W/m²K.
Will a loft conversion add value in London?
Yes. A well-designed loft conversion adding a bedroom and en-suite typically adds 15–25% to property value in London. In high-demand areas (Hackney, Islington, Wandsworth), the uplift often exceeds the construction cost. The net value depends heavily on the property's current bedroom count: adding a fourth bedroom adds more proportionally than adding a third.
Summary
A loft conversion is one of the most cost-effective ways to add significant living space to a London home. The right type depends on your property: terraced houses suit rear dormers, semi-detached and detached houses can do hip-to-gable, and inner London terraces in conservation areas often end up with a mansard.
Check head height and planning status before instructing an architect. Start the party wall process as soon as you have an outline scheme. Use a builder who works through building regulations and provides a completion certificate.
Budget 15–20% above the base construction cost to cover architect, structural engineer, building regulations, party wall surveyor fees, and contingency.
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