Loft Conversions in North London
Last updated: June 2026 · Figures verified against live North London quotes and current council planning records.
Quick answer: A loft conversion in North London costs £30,000–£100,000 depending on type, takes 6–10 weeks on site, and is usually permitted development — no full planning needed. We design, plan and build the whole thing for a fixed price. Free site visit: 020 3051 9430.
The loft is the cheapest extra room you'll ever build. You already own the space, the walls are up, and you're adding a bedroom, an office or a master suite without losing an inch of garden. The Extension Company designs, plans and builds loft conversions across North London — from our base in Cockfosters out to Barnet, Enfield, Southgate, Finchley, Muswell Hill and the towns in between. One fixed price, one team, one point of contact from first sketch to final sign-off. This page covers what we do, the five types of loft conversion and what each suits, why North London's 1930s semis are perfect for one type in particular, and how the process works.
What does The Extension Company actually do?
We're a design-and-build company, which means we handle the whole job under one roof rather than handing you off between an architect, a planning consultant and a separate builder. You get:
- Design — measured survey, structural layout, the staircase position, where the natural light goes, how the new space connects to the floor below.
- Planning — we establish your route (permitted development or full application), prepare the drawings, and submit the Lawful Development Certificate or householder application on your behalf.
- Building control — every loft conversion needs Building Regulations approval; we manage the inspections and the sign-off.
- Build — the structural work, dormer or roof alterations, insulation, the new staircase, plastering, electrics, plumbing and decoration, delivered by our build partner Pinegrove.
Three things define how we work. First, it's fixed price — you get an itemised quote after a free site visit and that's the number, not a starting bid that drifts upward. Second, it's end to end — no gaps between trades for you to project-manage. Third, it's local — we work North London every week, so we know how Barnet, Enfield and Haringey planning departments actually behave, not just what the rulebook says.
What are the types of loft conversion?
There are five main types, and the right one depends on your roof shape, your budget and how much headroom and floor space you need. Here's how they compare at 2026 North London build rates (all figures are all-in build cost, excluding VAT and professional fees):
| Type | What it suits | Typical cost | Planning |
|---|---|---|---|
| Velux / rooflight | Roofs that already have good headroom; lowest-cost option | £30,000 – £45,000 | Permitted development (almost always) |
| Rear dormer | Terraces and semis needing more headroom and floor space | £40,000 – £75,000 | Permitted development (usually) |
| Hip-to-gable + rear dormer | 1930s semis and detached homes with a hipped (sloping side) roof | £55,000 – £90,000 | Permitted development (usually) |
| L-shaped dormer | Victorian/Edwardian homes with a rear addition ("outrigger") | £65,000 – £95,000 | Permitted development (usually) |
| Mansard | Maximum space; period terraces; conservation areas | £60,000 – £100,000 | Often a full householder application |
A few words on each. A Velux (rooflight) conversion changes nothing on the outside of the roof — you simply add windows in the existing slope, insulate, board out and add a staircase. It's the cheapest and least disruptive option, but only works if your roof already has the headroom (you need roughly 2.2m–2.3m at the ridge). A rear dormer builds a box out of the back slope, creating full-height standing room and usable floor space — the workhorse conversion for terraces and semis. A hip-to-gable rebuilds a sloping side roof vertically to a gable end, reclaiming the volume that a hipped roof wastes; combined with a rear dormer it transforms a 1930s semi loft. An L-shaped dormer adds a second dormer over a rear back-addition, common on period London terraces, and gives the most floor space of the dormer options. A mansard rebuilds the whole rear (or both) roof slope at a steep angle with a near-flat top — the most space, the most expensive, and the most likely to need full planning, especially in a conservation area.
Why are North London's 1930s semis perfect for a hip-to-gable?
This is our strongest proof point, and it comes from real planning data rather than a sales pitch. When we scraped the approved-application records for the boroughs we work in, one scheme type stood out: the hip-to-gable loft conversion with a rear dormer is the single most-approved loft scheme in both Barnet and Enfield right now.
The reason is the housing stock. Vast swathes of Barnet, Enfield, Southgate and Winchmore Hill were built in the 1930s as semi-detached houses with hipped roofs — the type where the side of the roof slopes inward rather than rising to a flat vertical wall. That hip wastes a corner of usable loft volume. A hip-to-gable conversion rebuilds that sloping side vertically to a "gable" end, and when you add a rear dormer behind it you reclaim almost the entire roof space — enough for a double bedroom and an ensuite, or a generous master suite.
Three things make this work so reliably in North London:
- The roofs are built for it. A 1930s hipped semi is the textbook candidate; the geometry is exactly what a hip-to-gable is designed to fix.
- It's usually permitted development. Semi-detached houses get a 50m³ volume allowance under permitted development (40m³ for terraces), and a hip-to-gable plus dormer typically fits inside it — no full planning application required.
- The councils already approve it constantly. Because it's the most-approved loft scheme type in Barnet and Enfield, there's a clear, well-trodden path through the system — which means fewer surprises and a more predictable timeline for you.
If you own a 1930s semi in North London, a hip-to-gable with a rear dormer is very often the single best-value way to add a bedroom. We've built this transformation many times over — alongside larger projects like a 120m² home taken up to 360m², a 55m² garden room and an Edwardian conversion in Finchley.
How does the process work?
Four steps, one fixed price, one team.
- Free site visit & fixed quote. We come to you, measure up, check your roof type and headroom, talk through what you want the space to do, and follow up with an itemised, fixed-price quote — no obligation. This is also where we confirm whether you're permitted development or need a full application.
- Design & planning. We produce the drawings and submit your planning route on your behalf — a Lawful Development Certificate (£274) to prove permitted development, or a full householder application (£548 plus the £91.02 portal charge) if you're beyond PD or in a conservation area. In parallel we handle the structural engineer's calculations (£500–£1,500).
- Building Regulations & party wall. Every loft conversion needs Building Regulations approval (~£500–£1,500). On a terrace or semi you'll usually need a party wall agreement with the neighbours you share a wall with — that costs £700–£2,500 per neighbour if surveyors are appointed, or nothing if your neighbour consents in writing within 14 days.
- Build. Our build partner Pinegrove carries out the work, typically 6–10 weeks on site. Most of the structural work happens through the roof, so you can usually stay living in the house throughout. We hand over a finished, Building-Control-signed-off room.
Which areas of North London do we cover?
We design and build loft conversions right across North London. We work in:
Not listed? We cover the surrounding towns too — call and ask.
How much will mine cost?
The honest answer depends on your roof type, the conversion type and your spec. As a guide, a loft conversion adds roughly 15–20% to a property's value — a widely-reported figure across the UK market, though never a guarantee on any individual home. For the full breakdown of prices by type, by square metre and by house type, read our loft conversion cost guide, or get an exact number for your loft with the Extension Builder.
Frequently asked questions
Do I need planning permission for a loft conversion in North London? Usually not. Most loft conversions are permitted development — you get a 40m³ volume allowance on a terrace and 50m³ on a semi-detached house, with no roof extension beyond the front plane. We recommend a Lawful Development Certificate (£274) to prove it on paper. You'll only need a full householder application (£548 plus a £91.02 portal charge) if your scheme exceeds permitted development or you're in a conservation area or under an Article 4 direction.
How long does a loft conversion take? Typically 6–10 weeks on site once we start. Design and planning before that add a few weeks, depending on your route — a Lawful Development Certificate is quicker than a full application.
Will I need a party wall agreement? On most North London terraces and semis, yes — you share a wall with a neighbour. Budget £700–£2,500 per affected neighbour if surveyors are appointed, or £0 if your neighbour gives written consent within 14 days of being served notice. We handle the process for you.
What's the best loft conversion for a 1930s semi? A hip-to-gable with a rear dormer, almost every time. It rebuilds the wasted sloping side of the roof and adds a box dormer at the back, reclaiming nearly the whole loft — usually within permitted development. It's the most-approved loft scheme in Barnet and Enfield for exactly this reason.
Can I stay in my house during the build? In most cases, yes. The bulk of a loft conversion is carried out through the roof, so daily disruption to the floors below is limited. We'll tell you at the site visit if there's any stage where that changes.
Thinking about converting your loft? Get a fixed, itemised price for your exact roof with the Extension Builder, or book a free site visit and we'll measure up and tell you your planning route on the spot. Call 020 3051 9430 to talk it through with the team.