If you're researching what a loft conversion costs in London, the honest answer is: it depends almost entirely on the type of conversion, and London rates run well above the national figures you'll see quoted elsewhere. A simple rooflight job and a full mansard are different builds with a £70,000 gap between them. This guide gives real 2026 ranges for North London — by type, by square metre, and by house type — then itemises every extra cost (building control, party wall, structural engineer, VAT) so you can budget properly rather than be surprised later. Pine Grove's own team builds loft conversions across North London, Barnet, Enfield and Southgate included, and these are the ranges to budget from. If you searched for loft extension costs, you are in the right place: a loft extension is a loft conversion that enlarges the roof, and it is priced in the same table. What each type involves, and how the planning side works, is on our loft conversions and loft extensions page.
What does a loft conversion cost by type in 2026?
Type is the single biggest driver of price. These are realistic all-in build ranges for North London (labour and materials to a good finish), excluding VAT and professional fees:
| Type | What it is | North London cost (2026) |
|---|---|---|
| Velux / rooflight | Windows in the existing roof slope, insulate, board out, staircase | £30,000 – £45,000 |
| Rear dormer | A box built out of the rear roof slope for full standing height | £40,000 – £75,000 |
| Hip-to-gable + rear dormer | Rebuilds the sloping side roof to vertical, plus a rear dormer | £55,000 – £90,000 |
| L-shaped dormer | Two dormers, including over a rear back-addition | £65,000 – £95,000 |
| Mansard | Whole rear roof slope rebuilt steeply with a near-flat top | £60,000 – £100,000 |
In premium and conservation-controlled areas — parts of Highgate, Hampstead and Muswell Hill — the same conversions run higher, often £3,500–£5,000+ per m², because of higher-spec materials, tighter design control and more careful work.
What does a loft extension cost?
The same as the loft conversion types that extend the roof, because that is what a loft extension is. A rooflight conversion fits out the existing roof space and adds no volume. A dormer, hip-to-gable, L-shaped dormer or mansard builds new roof, and those four are what most people mean by a loft extension:
- Rear dormer loft extension: £40,000–£75,000
- Hip-to-gable with a rear dormer: £55,000–£90,000
- L-shaped dormer: £65,000–£95,000
- Mansard loft extension: £60,000–£100,000
So a loft extension in London costs £40,000–£100,000 to build in 2026, excluding VAT and fees, against £30,000–£45,000 for a rooflight conversion that leaves the roof shape alone. The more new roof structure there is, the more it costs. Our guide to loft extensions and how they differ from a loft conversion explains the terms.
What is the cost per square metre?
The cleanest way to sanity-check any loft quote is per square metre:
- National UK average: roughly £1,250–£2,000 per m² for a loft build.
- North London: £2,800–£4,200 per m² for the build, excluding VAT and fees.
- Premium / conservation North London: £3,500–£5,000+ per m².
London sits well above the national figure for the same three reasons every London build does: higher labour and trades rates, period properties that need slower and more careful work, and tight-site access and logistics. So a dormer adding around 18m² of usable floor works out at roughly £50,000–£75,000 to build before VAT and fees, the upper part of the dormer range in the table above.
What's included in the price — and what isn't?
A headline loft build price usually covers the structural work, the dormer or roof alterations, insulation, plastering, the new staircase, basic electrics, plumbing first-fix and a standard finish. It usually does not include:
- VAT — most residential loft work is standard-rated at 20% from a VAT-registered builder. Always check whether a quote is inclusive.
- Professional fees — planning, structural engineering and building control (itemised below).
- The ensuite fit-out — sanitaryware, tiling and a shower enclosure are typically £4,000–£10,000 on top.
- Bespoke extras — a feature staircase, rooflights upgraded to large glazed gable windows, fitted wardrobes, high-end flooring and underfloor heating.
When you compare two quotes, compare what's in them. A cheaper headline number is very often cheaper because it leaves more out.
How does cost vary by house type?
The same conversion costs different amounts on different houses, because the roof and the structure differ:
| House type | Typical conversion | Notes on cost |
|---|---|---|
| Victorian/Edwardian terrace | L-shaped dormer or mansard | Solid walls and a rear back-addition; more complex, often £65k–£100k |
| 1930s semi (hipped roof) | Hip-to-gable + rear dormer | Suits the hipped roof; usually £55k–£90k and often permitted development |
| Detached | Hip-to-gable + dormer or large dormer | Easier access, bigger volume allowance; spec drives the number |
| Bungalow | Full dormer or rooflight | Often a big proportional gain in floor space |
Two patterns matter for North London. Victorian and Edwardian terraces (common in Winchmore Hill, Crouch End and Palmers Green) have solid brick walls and a rear outrigger, which suits an L-shaped dormer or a mansard but adds structural and detailing cost. 1930s semis (the bulk of Barnet, Enfield and Southgate) have hipped roofs built for a hip-to-gable, which squares off the side slope so a rear dormer can run the full width. The Barnet and Enfield loft guides list recent loft decisions from each council.
What are the extra costs to budget for?
Beyond the build, a properly run loft conversion carries these costs. None of them are optional surprises if you plan for them up front:
- Building Regulations: always required for a loft conversion — budget ~£500–£1,500 for the application and inspections.
- Structural engineer: £500–£1,500 for the calculations and steel/beam design that every loft needs.
- Party wall agreement: common on terraces and semis — £700–£2,500 per neighbour if surveyors are appointed, or £0 if your neighbour consents in writing within 14 days of notice.
- Staircase: a new staircase up to the loft is part of the build, but a bespoke or feature stair adds cost over a standard flight.
- Ensuite: adding a bathroom to the loft typically adds £4,000–£10,000 for plumbing, sanitaryware and tiling.
- VAT: 20% on top from a VAT-registered builder, unless the quote states it's inclusive.
As a rough rule, fees and the extras above can add 10–20% over the headline build cost, so build them into your budget from day one.
What's the planning route — and what does it cost?
Most loft conversions are permitted development, which means no full planning application:
- Terraced house: 40m³ volume allowance.
- Semi-detached / detached: 50m³ volume allowance.
- The new roof can't rise above the existing ridge or extend beyond the roof slope facing the road.
Even when you're permitted development, we recommend a Lawful Development Certificate (£274) — it's the document that proves on paper your loft was lawful, which matters when you sell. You'll only need a full householder application (£548 from April 2026, plus a £91.02 Planning Portal charge) if your scheme exceeds permitted development, or your home is in a conservation area or under an Article 4 direction. Mansards, in particular, more often need the full route. For the complete fee picture, see our planning fees guide.
What does a loft conversion and an extension together cost?
There is no separate rate for doing both. Budget each part from its own range: the loft from the table at the top of this guide, and a single-storey rear extension at £45,000–£90,000 to build (£2,800–£4,200 per m², excluding VAT and fees), as set out in our North London extension cost guide. Building them as one project means one design, one scaffold and one party wall process, and the two use separate permitted development allowances, so the loft does not use up the extension's limits. How we run a loft and an extension as one project is covered on the loft conversions and loft extensions page.
How do I get an accurate figure?
Online ranges only get you so far — the price of your loft depends on your roof type, the conversion type, your spec and your borough. Two ways to get a real number:
- The Extension Builder prices your exact loft and tells you your planning route in about 60 seconds.
- A free site visit — we come to you, measure up, check your roof and headroom, and follow up with a fixed, itemised quote with no obligation.
For the full picture of how loft conversions and loft extensions work — the types, the permitted development limits, the process and the areas we cover — see the loft conversions hub.
Want a real price for your loft? Get a fixed, itemised figure for your exact roof with the Extension Builder, or book a free site visit and we'll measure up and confirm your costs and planning route. Call 020 3051 9430 to talk it through with the team.