How much does a lean-to extension cost in north London? (2026)

A lean-to extension is a single-storey extension with a roof that slopes down from the wall of the house. In north London it's most often a side return infill on a Victorian terrace, or a kitchen extension across the back. Our recent single-storey extension quotes have come in at roughly £56,000 to £90,000 including VAT in 2026.

The lean-to roof itself isn't what decides the price. The foundations, the steel beams and the opening into the house cost the same whatever roof goes on top. Below is where the money goes, what the roof changes, and how to tell whether a quote is fair.

What a lean-to extension costs

We price a lean-to the same way as any single-storey extension, because it's the same build with a different roof. These are the figures we publish.

• Our recent quotes. Our single-storey extension quotes from 2025 and 2026 have come in between about £56,000 and £90,000 including VAT.
• Our guide prices. A single-storey rear extension is £55,000 to £110,000 and a side return infill is £45,000 to £85,000, including VAT, on a mid-terrace. A semi or a detached house costs more, because there's more to build.
• Public London guides. HomeOwners Alliance gives £2,300 to £3,400 per square metre for a single-storey extension in London, and Resi gives £1,900 to £3,000 for London and the South East. Most of these figures are before VAT and before the kitchen.

A typical extension on a north London terrace adds 15 to 20 square metres of floor. That's why a lean-to over a side return, which is narrow, often sits near the bottom of these ranges, and a deep lean-to across the full width of a semi sits near the top.

Our guide on what a rear extension costs in north London goes through the same numbers in more detail, with the sources.

What the lean-to roof changes

The look and the light. A lean-to roof slopes from just under the first floor windows down to the new outside wall. Roof windows in the slope bring light deep into the room, and the ceiling follows the slope, so the room feels taller at the house end.

The cost. A pitched roof with roof windows generally costs more than a flat roof with roof lights. It needs rafters, a wall plate fixed to the house, tiles or slate to match, flashing where it meets the wall, and the roof windows themselves. If budget is tight, a flat roof is usually the cheaper choice. When we build a flat roof it's a warm roof finished in EPDM rubber.

The height at the house. The top of the slope has to sit below the first floor windows and keep a sensible pitch for the tiles. On some houses that leaves the outside wall too low for the room you want, and a flat roof works better. We check this at the survey.

Lean-to kitchen extensions and side returns

Most lean-to extensions in north London are kitchens. On a Victorian or Edwardian terrace the classic job is filling the narrow side return beside the back addition, with a lean-to roof sloping down to the boundary wall and glass in the slope. That gives a kitchen the full width of the house.

Our guide on whether a side return infill is worth it explains when that's the right move and when a rear extension is better.

Planning and the rules on height

Many lean-to extensions are permitted development, so no planning application is needed. The main limits for a single-storey rear extension are these.

• No more than 3m beyond the original rear wall, or 4m on a detached house. Up to 6m and 8m with prior approval from the council.
• No more than 4m high.
• Within 2m of a boundary, the eaves can be no higher than 3m.
• Side extensions aren't permitted development in a conservation area.

A lean-to suits these rules well, because the roof is lowest at the boundary end. Our guide do I need planning permission for a rear extension in Enfield? explains the options and the council fees. If you share a wall with a neighbour, read our guide to party wall notices too.

How long it takes

Our guide build times are 12 to 16 weeks on site for a single-storey rear extension and 10 to 14 weeks for a side return infill. Drawings and approvals come before that. The new shell goes up outside your existing wall first, and we only break through once it's weathertight.

The fees on top of the build

These usually sit outside the build price. The figures are from Homebuilding & Renovating and HomeOwners Alliance, as cited in our rear extension guide.

• Architect or designer, from about £4,500 to £6,500
• Structural engineer, about £1,500 to £2,500
• Building Regulations, about £1,200
• A party wall agreement, about £1,000 per neighbour

How to get a fair price

• Ask for the roof options side by side. Lean-to and flat, on the same quote, so you can see the real difference.
• Decide the glazing and the kitchen first, or give every builder the same allowance. They move the price more than the roof does.
• Ask what's excluded. A cheaper quote often just leaves more out.
• Get a survey before drawings. Trees, drains and the height under the first floor windows are all visible on site.

Our guide what a rear extension really costs explains why the first metre is the expensive one.

Paying for it

We don't take a deposit. You pay weekly in arrears, with an invoice every Friday for the work done that week. You never pay for work that hasn't happened yet.

See one we built

Our rear extension in Palmers Green is a kitchen and dining room across the back of a terrace, with steel beams and black-framed doors to the patio. You can see the rest of our work on the extensions page.

Sources and dates

• Promaester, our own 2025-26 single-storey extension quotes, shown as a rounded range only, and the guide prices on our extensions page. Prices include VAT at 20%.
• HomeOwners Alliance, House extension costs, updated 30 July 2026, checked 5 October 2026.
• Resi, How much does a rear extension cost in the UK?, published 13 February 2026, checked 5 October 2026.
• Homebuilding & Renovating, How much does an extension cost in 2026?, updated 19 February 2026, checked 5 October 2026.
• Planning Portal, Extensions: permitted development, checked 5 October 2026.

These are guide figures, not a quote. For a real number, book a free site visit. We'll look at the back of the house, the height you have to work with and the access, and give you a fixed, itemised quote.