Loft conversions & loft extensions in North London and across London

A loft conversion adds a room without giving up a square metre of garden: the floor area is already yours and the walls are already standing. North London's housing suits lofts well. Large parts of Barnet and Enfield were built between the wars as streets of semi-detached houses under hipped roofs, and a hipped roof is exactly what a hip-to-gable conversion is designed to fix. Haringey's Victorian and Edwardian terraces, from Muswell Hill down to Crouch End, have no hip to square off, so there the loft goes into a rear dormer, or an L-shaped dormer that carries on over the back addition. This page covers the five types of loft conversion and loft extension with their 2026 costs, the permitted development limits, when planning permission is needed, the 10-year rule, doing a loft conversion and extension as one project, and what Enfield, Barnet and Haringey have decided on lofts.

Loft conversion or loft extension: what is the difference?

Nothing that changes the job. Builders, planners and building control officers say loft conversion; a lot of homeowners say loft extension. The one distinction worth knowing is whether the roof gets bigger:

• A rooflight (Velux) conversion fits out the space already under the tiles. The roof keeps its shape and no volume is added.
• A dormer, hip-to-gable, L-shaped dormer or mansard builds new roof to make headroom and floor area. That is why people call them loft extensions.

Most North London houses need the second kind, because a Victorian terrace or a 1930s semi rarely has enough standing height under the roof it was built with. We use both terms on this page for the same work. Our guide to loft extensions and how they differ from a loft conversion goes through the difference in full.

What a loft and extension company does

The Extension Company is the extensions studio of Pine Grove Design & Build Ltd. We design loft conversions and house extensions, take them through planning and building control, and in North London Pine Grove's own team builds them. That is one firm from first survey to sign-off, instead of an architect, a planning consultant and a separate builder passing the job between them. 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. On North London lofts that is Pine Grove's own team on site.

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: a North London loft stays with Pine Grove's own team from survey to handover, and each scheme is drawn to the rules of the council deciding it, whether that is Barnet, Enfield or Haringey.

Types of loft conversion and loft extension

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 is how they compare at 2026 London build rates (all figures are all-in build cost, excluding VAT and professional fees):

Velux or rooflight loft conversion

Nothing changes on the outside of the roof: windows go into the existing slope, the loft is insulated and boarded out, and a staircase goes in. It is the cheapest and least disruptive option, but it only works if the roof already has the headroom, roughly 2.2m–2.3m at the ridge.

Rear dormer loft extension

A dormer builds a box out of the back slope, giving full standing height and usable floor space. It is the workhorse loft extension for terraces and semis: in the council figures further down this page, rear dormers outnumber hip-to-gables in all three boroughs. Our guide to dormer loft conversions in London covers the design choices.

Hip-to-gable loft conversion

A hip-to-gable rebuilds a sloping side roof vertically to a gable end, reclaiming the volume a hipped roof wastes. Combined with a rear dormer it turns the loft of a 1930s semi into a double bedroom with an ensuite.

L-shaped dormer

A second dormer runs over the rear back addition and joins the main rear dormer. It is common on period London terraces and gives the most floor space of the dormer options, often two rooms or a bedroom and a bathroom.

Mansard loft extension

A mansard rebuilds the whole rear roof slope (or both slopes) at a steep angle with a near-flat top. It gives the most space, costs the most, and is the most likely to need a planning application, especially in a conservation area.

Our guide to the types of loft conversion walks through each in more detail.

Which loft suits which North London house?

It comes down to the roof, and North London has two roofs above all.

1930s semis: hip-to-gable with a rear dormer

Much of Oakwood, Southgate and East Barnet went up between the wars as bay-fronted semis with hipped roofs: the side of the roof slopes inward instead of rising to a vertical gable wall, and that slope eats a wedge of the loft. A hip-to-gable conversion builds the side up to a gable end. Put a rear dormer behind it and nearly the whole roof becomes usable, with room for a double bedroom and an ensuite, or a main bedroom suite. A hipped semi, or an end-of-terrace with a hipped end, is the house this conversion was designed for. The area guides where this roof leads are loft conversions in Barnet, loft conversions in Cockfosters, loft conversions in Southgate, loft conversions in Edgware and loft conversions in Hendon.

Victorian and Edwardian terraces: rear and L-shaped dormers

A mid-terrace has no hip to convert, so a rear dormer does the work, and where the house has a two-storey back addition a second dormer over it makes the L-shape. A mansard is the option when the whole rear slope needs rebuilding for height. The area guides that cover terraces and period roofs in most detail are loft conversions in Enfield, loft conversions in Palmers Green, loft conversions in Winchmore Hill, loft conversions in Finchley and loft conversions in Muswell Hill.

Further down this page are the councils' own decisions on rear dormers and hip-to-gable roof extensions, with the refusals alongside the approvals, taken from the GLA Planning London Datahub. They cover every applicant's schemes, not ours, but they show how often Enfield, Barnet and Haringey see this work and how often each says no. The area guides go into each town's roofs and its recent loft decisions, and our page on loft conversions in North London brings them together borough by borough. The wider picture for ground-floor work is in our guide to house extensions across North London.

Permitted development for loft conversions: the 40m³ and 50m³ limits

Most loft conversions and loft extensions on houses are permitted development: national rules grant the permission, so no planning application is made. The limits for roof extensions are:

• Volume. Up to 40m³ of added roof space on a terraced house and 50m³ on a semi-detached or detached house. An end-of-terrace house counts as terraced. Any earlier addition to the roof counts towards the total.
• Front slope. Nothing may extend beyond the plane of the roof slope that faces the road, so a front dormer is not permitted development. Rooflights in the front slope are allowed, provided they project no more than 15cm from it.
• Height. No part of the work may be higher than the highest part of the existing roof.
• Eaves. A dormer sits at least 20cm back from the eaves, measured along the roof slope, where practicable. A hip-to-gable is exempt.
• Materials. Similar in appearance to the existing house.
• Side windows. Obscure-glazed, and fixed shut unless the opening part is more than 1.7m above the floor.
• No balconies. No balcony, veranda or raised platform. A Juliet balcony with nothing to step out onto is normally acceptable.
• Houses only. Flats and maisonettes have no permitted development rights for roof extensions.

On a hipped 1930s semi, a hip-to-gable with a rear dormer normally fits inside the 50m³. Permitted development does not mean no paperwork: we recommend a Lawful Development Certificate (£274), which is the council's written confirmation that the loft is lawful and the document a buyer's solicitor asks for, and every loft needs Building Regulations approval whatever its planning route. The rules are set out in full in our guide to the permitted development rules for loft conversions.

When does a loft conversion need planning permission?

A householder planning application (£548, plus the £91.02 Planning Portal charge) replaces the certificate when:

• The house is in a conservation area. Roof enlargements are not permitted development there. A dormer or hip-to-gable in the Muswell Hill, Monken Hadley or Winchmore Hill Green conservation areas, for example, needs an application.
• An Article 4 direction covers the street. The council has withdrawn the permitted development right, so check before counting on a certificate.
• The property is a flat or maisonette.
• The design breaks a limit. A front dormer, a roof raised above the existing ridge, or more volume than the 40m³ or 50m³ allowance.
• It is a mansard. Not always, but often: a mansard changes the roof more than the limits usually allow.

A listed building needs listed building consent as well. Councils aim to decide a householder application within eight weeks of validating it. Our guides to planning permission for a loft conversion, planning permission for a dormer and extending in a conservation area cover each case.

The 10-year rule: lofts built without permission

The 10-year rule is the time limit on planning enforcement. In England a council has 10 years from the date building work was substantially completed to take enforcement action against work that needed planning permission and did not have it. The limit was four years until 25 April 2024, and work substantially completed before that date keeps the four-year limit. Once the limit has passed the work is immune from planning enforcement, and a lawful development certificate for existing work (£548) records that in writing.

It is not something to build on. Ten years is a long time to own a loft that a buyer's solicitor will question, and a council can apply to act after the limit where the work was deliberately concealed. The planning clock also does nothing for Building Regulations, which have a 10-year limit of their own: since 1 October 2023 a council in England can require work that breaks the regulations to be altered or removed up to 10 years after it was completed. If you have bought or inherited a loft with missing paperwork, our guides to lawful development certificates and building without planning permission set out how to put it right. For a new loft, the £274 certificate before work starts avoids the question altogether.

Loft conversion and extension in one project

Plenty of North London houses need both: bedrooms in the roof and a bigger kitchen at the back. A loft conversion and extension can be designed as one scheme and built as one job, and in North London Pine Grove's own team builds both on one programme. What doing them together changes:

• One design. The loft stair, the steelwork, the drainage, the boiler flue and the soil pipe are planned once for the finished house, so the second job does not start by undoing part of the first.
• One site set-up. One scaffold and one run of skips serve the roof and the ground floor.
• One party wall process. On a terrace or semi, the notices for the loft and the extension can be served together.
• Separate planning allowances. A loft uses the roof allowance of 40m³ or 50m³. A single-storey rear extension has its own limits: 3m beyond the original rear wall of a terraced or semi-detached house, or 4m on a detached one, stretched to 6m or 8m through prior approval (£249, with the council given 42 days to decide). One does not use up the other, so outside a conservation area many houses can do both without a planning application. Where permission is needed, one householder application (£548) can cover the whole scheme.

Budget each part from its own range: the loft from the table above, 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. If the budget will not cover both at once, stage them, but have the whole house drawn first so the loft stair and the steelwork allow for the extension to come. Our loft conversion vs extension comparison helps with which to build first, and the ground-floor side is covered on our single-storey rear extension and kitchen extension pages.

How does the process work?

Four steps, one fixed price, one team.

1. 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.
2. 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).
3. 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.
4. Build. In North London, Pine Grove's own team builds it, typically in 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.

What our loft clients say

Shown word for word from our Google profile, rated 5.0 from 16 reviews. Read them all on Google.

Why winter is loft season

Most people assume building work stops for winter. Lofts are the exception, and the trade knows it: once the roof structure is weathertight — the first week or two of the build — almost everything else happens inside. Insulation, boarding, the staircase, first fix, plastering and decoration carry on regardless of what January is doing outside. That makes autumn the smart time to start the design and planning route: survey and drawings in the autumn, permission or certificate in hand by early winter, built through the cold months, and a finished room before spring — while extension projects queue for the ground to dry out. If you want the loft done for next summer, the site visit that starts it belongs on this side of Christmas.

How much does a loft conversion or loft extension cost?

Between £30,000 and £100,000 to build in 2026, excluding VAT and professional fees: from £30,000–£45,000 for a rooflight conversion to £60,000–£100,000 for a mansard, with each type priced in the table near the top of this page. That works out at about £2,800–£4,200 per m² in North London, and £3,500–£5,000+ per m² in premium and conservation areas. On top come VAT at 20%, Building Regulations (about £500–£1,500), the structural engineer (£500–£1,500), party wall surveyors where a neighbour does not consent (£700–£2,500 per neighbour) and the ensuite fit-out (£4,000–£10,000). A loft conversion is widely reported to add roughly 15–20% to a property's value, though that is a market figure and never a guarantee on any individual home. For the full breakdown by type, by square metre and by house type, read our guide to loft conversion and loft extension costs in London, or get a number for your own loft with the Extension Builder.

The loft conversion library

Everything we publish on lofts, in one place — each guide goes deeper on its own question:

• Rules and permissions: do I need planning permission for a loft conversion? · the permitted development rules for lofts in full · do you need planning permission for a dormer? · lawful development certificates · our complete planning permission guide
• Design and build: the types of loft conversion · dormer loft conversions in London · loft conversion ideas · loft extensions and how they differ
• Practicalities: how long a loft conversion takes · building regulations for lofts · party walls for lofts · how much value a loft adds
• The big decision: loft conversion vs extension for the up-or-out question

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Thinking about a loft conversion, a loft extension, or a loft and an extension together? 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.