Attic Conversion London: Is It the Same as a Loft Conversion?

By Anderson & the LCCL Construction Team | Finchley N3

People searching for an attic conversion in London are often unsure whether they are asking for something different from a loft conversion. They are not. In British building practice the two words describe the same project: turning the unused roof space of a house into a habitable room, with a proper staircase, insulation, fire protection and building control sign-off. “Attic” tends to be used by people who grew up outside the UK, or who picture a smaller space under a steep roof; “loft” is the trade term and what the councils and building regulations use.

This guide covers what an attic needs to be convertible, the 2026 costs inclusive of VAT, which conversion type suits which London roof, and the planning and regulations that apply — from our team at LCCL Construction in Finchley N3, converting roofs across London and Hertfordshire since 2004.

An attic bedroom under the original roof slope — rooflights, eaves storage and a new floor are the essentials.

Attic conversion vs loft conversion: the only differences that matter

There is no legal, planning or building-regulations distinction between an attic and a loft. What people usually mean when they say “attic” is one of three things: a small space under a pitched roof (a rooflight or Velux conversion), a space in a flat rather than a house (which changes the planning route — more on that below), or simply a storage room they want to make habitable. All three are loft conversions in the eyes of your council and your building inspector.

The practical questions are the same whichever word you use: is there enough head height, can a staircase land without wrecking the floor below, and what does the roof structure allow?

Does my attic qualify? The 2.2 m rule and what else we check

The first measurement is head height at the ridge: 2.2 m from the top of the existing ceiling joists to the underside of the ridge is the practical minimum for a simple conversion, because the new floor and insulation take up to 300 mm and you still want 2 m of clear height in the finished room. Victorian and Edwardian terraces in Islington, Hackney and Haringey usually pass; 1930s semis in Barnet and Enfield usually pass; shallow 1960s–70s trussed roofs often do not without raising the ridge or rebuilding the roof.

• Roof structure. Traditional cut roofs (rafters and purlins) convert easily. Trussed roofs need the webs removing and new steel or timber beams designed by a structural engineer.

• Staircase landing. A loft stair needs 2 m headroom over the pitch line; on a terrace it normally lands over the existing stair, taking a slice of a back bedroom.

• Fire escape. A three-storey house needs a protected stair enclosure, fire doors to habitable rooms and mains-wired interlinked alarms. Open-plan ground floors need a sprinkler or a redesign.

• Water tanks and chimneys. Tanks usually go (combi or unvented cylinder); chimney breasts can stay as features or be removed with proper support.

Which conversion type suits a London attic

The type you need is set by the roof you have and the space you want. In cost order, inclusive of VAT and design & build:

• Velux / rooflight conversion — £32,400 – £50,400 inc. VAT (£27,000 – £42,000 + VAT). Roof shape unchanged; rooflights in the slope; best when the ridge is high. Almost always permitted development.

• Rear dormer — £56,400 – £80,400 inc. VAT (£47,000 – £67,000 + VAT). A box extension on the rear slope giving full head height across most of the room. The London default for terraces and semis.

• L-shaped dormer — £58,800 – £98,400 inc. VAT (£49,000 – £82,000 + VAT). Dormer over the main roof and the back addition of a Victorian terrace — two rooms and a bathroom.

• Hip-to-gable — £60,000 – £114,000 inc. VAT (£50,000 – £95,000 + VAT). The sloping side of a semi or end-of-terrace rebuilt as a vertical gable; usually paired with a rear dormer.

• Mansard — £76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT). The whole rear slope rebuilt at 72°; the most space and nearly always a planning application.

Inner London boroughs add 10–15% to these figures, Central London 20–25%, and Hertfordshire comes in about 5% lower. Our loft conversion cost guide breaks each band down further, and the types guide compares them room by room.

Attic conversions in flats: the planning rule that catches people out

If your “attic” sits above a flat rather than a house, the project is still a loft conversion — but flats have no permitted development rights, so every roof alteration needs a full planning application, and you will need the freeholder's consent and usually a licence to alter. Allow eight weeks for the decision and build the legal step into the programme.

Our Weech Road project in Hampstead NW6 is exactly this situation: a top-floor flat converted into a penthouse duplex, with the kitchen and living space relocated up into the new loft level, an extra bedroom, two new bathrooms and views across to the Shard and the London Eye. It won LCCL the LABC Building Excellence Award 2026 — London Regional Winner for Best Conversion or Alteration to an Existing Home. See the Weech Road project.

Planning permission and building regulations

For a house, most attic conversions are permitted development provided the added roof volume stays within 40 m³ on a terrace or 50 m³ on a semi or detached house, nothing projects above the existing ridge, and the dormer sits back from the eaves. We obtain a Lawful Development Certificate on every permitted-development loft — it is the document your buyer's solicitor will ask for. Conservation areas with an Article 4 direction, and any front-facing dormer, need a householder planning application instead. Our loft planning permission guide covers the detail.

Building regulations apply to every conversion without exception: structure, fire, insulation, stairs, ventilation and electrics are inspected and certificated. On a terrace or semi the Party Wall Act also applies, because the new floor beams bear into the shared wall — notices go to the neighbours two months before work starts.

Anderson's advice

“Semi-detached hip to gable + rear dormer combos almost always fall under permitted development, but neighbours often object informally.” The planning route being automatic does not mean the street agrees with you. Knock on the door before the scaffold goes up, show the drawings, explain the party wall notice before it lands on the mat. It costs nothing and it is the difference between a quiet twelve weeks and a formal complaint to the council that delays everyone.

Why LCCL for an attic conversion

As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — we design and build every loft with our own architectural, structural and construction teams under one contract. Survey, drawings, structural calculations, the planning or Lawful Development route, building control, party wall notices and the build itself are all in the price — with a 12-month workmanship guarantee and a 10-year LABC structural warranty.

What our clients say

“I highly recommend LCCL Construction. They completed our loft conversion to a very high standard, and the quality of workmanship throughout was excellent. The team was professional, reliable, and completed everything within a very reasonable timeframe. They were also fairly priced, which made the whole experience even better. We’re really pleased with the finished result and would definitely use them again.”

— Alina A, Google Reviews

Frequently asked questions

Is an attic conversion cheaper than a loft conversion?

No — they are the same project. What is cheaper is a rooflight (Velux) conversion compared with a dormer, because the roof shape stays as it is. If your “attic” has a high ridge, that route starts at £32,400 inc. VAT (£27,000 + VAT).

How long does an attic conversion take?

A rooflight conversion is typically 4–6 weeks on site, a rear dormer 8–10 weeks, an L-shaped or hip-to-gable 10–12 weeks and a mansard 12–16 weeks — after 8–16 weeks of design, approvals and party wall process.

Can I convert the attic myself and get it signed off later?

You can apply for regularisation, but inspectors will open up floors and walls to check the structure and fire protection, and work that fails has to be redone. For a room that will be slept in, three storeys up, it is not a place to save on the inspection route.

Get a figure for your own project

To book a free 30-minute consultation call about your attic conversion, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.

One team, one contract, one point of contact — from first sketch to handover.