Loft Conversion Guide UK — Costs, Types & Planning
A loft conversion is one of the best ways to add space and value to your home without moving. On average, a well-done loft conversion adds 15–20% to your property value. Here is everything you need to know before starting.
Is Your Loft Suitable?
Not every loft can be converted. The key measurements:
Head height
You need at least 2.2 metres from the top of the ceiling joists to the underside of the ridge beam. Measure at the highest point. Below 2.2m may still work with a dormer or hip-to-gable conversion.
Roof structure
Traditional rafters (pre-1960s homes) are easier to convert. Trussed roofs (most homes built after 1960) have W-shaped supports that need to be replaced with steel beams — more expensive but still doable.
Staircase position
You need space for a proper staircase (not a ladder). This usually means losing some space from the room below, typically from the landing, a bedroom, or above a hallway.
Types of Loft Conversion
Velux / Rooflight
£20,000 – £35,000
The simplest and cheapest option. The existing roof shape is not changed — windows are installed into the slope. Works if you already have enough head height. Usually does not require planning permission.
Dormer
£30,000 – £55,000
A box-shaped extension built out from the slope of the roof, creating full-height space with flat ceiling and vertical walls. The most popular type. A rear dormer usually falls under permitted development. An L-shaped dormer is common on Victorian terraces.
Hip-to-Gable
£40,000 – £65,000
The sloped side of a hipped roof is built up into a flat gable wall, creating much more usable space. Common on semi-detached and detached houses. Often combined with a rear dormer for maximum space.
Mansard
£50,000 – £75,000
The roof slope is replaced with a near-vertical wall (at 72 degrees) with a flat roof on top. Creates the most space but is the most expensive and almost always requires planning permission. Common in London terraces.
Planning Permission
Most loft conversions fall under permitted development, meaning you do not need planning permission, provided you meet these conditions:
- Volume does not exceed 40 cubic metres (terraced) or 50 cubic metres (detached/semi)
- Does not extend higher than the existing roof ridge
- Does not extend beyond the plane of the existing roof facing a highway
- Materials are similar to the existing house
- No verandas, balconies, or raised platforms
- Side-facing windows are obscure-glazed and fixed shut (or top-opening only)
You DO need planning permission if: You live in a conservation area, listed building, AONB, or National Park. You are building a mansard. Your permitted development rights have been removed. Always check with your local council. See our planning permission guide for more detail.
Building Regulations
Every loft conversion needs building regulations approval, regardless of whether you need planning permission. This is non-negotiable. Building control will inspect:
Structural integrity: The floor must support the weight of the new room. Steel beams are usually required.
Fire safety: A protected escape route (fire doors, fire-rated walls, smoke alarms). If the loft is more than 4.5m above ground level, an escape window is required.
Insulation: Minimum thermal performance standards for walls, floor, and roof.
Staircase: Must comply with minimum width, headroom, and pitch requirements.
Electrics and plumbing: Must be installed by certified professionals (Part P for electrics).
What Does It Cost?
| Type | Cost (2026) | Value added |
|---|---|---|
| Velux / rooflight | £20,000 – £35,000 | 10–15% |
| Rear dormer | £30,000 – £55,000 | 15–20% |
| Hip-to-gable + dormer | £40,000 – £65,000 | 15–20% |
| Mansard | £50,000 – £75,000 | 20–25% |
Costs vary significantly by location. London prices can be 30–50% higher than the North.
How Long Does It Take?
A typical loft conversion takes 6–12 weeksfrom start to finish. A simple Velux conversion can be done in 4–6 weeks. A mansard or hip-to-gable with dormer takes 10–14 weeks. Most of the work is done from scaffolding outside, so disruption inside the house is relatively low until the staircase stage.
Tips for Getting It Right
Get 3+ quotes from specialists, not general builders. Ask to see previous loft conversions they have done.
Check party wall obligations. If you share a wall with a neighbour (terraced or semi), you need a party wall agreement. Budget £700 – £1,000 per neighbour for the surveyor.
Plan the staircase carefully. The staircase position affects both floors. Get this right before anything else.
Include an en-suite if possible. A loft bedroom with en-suite adds more value than a loft bedroom without.
Get a completion certificate. When building control signs off, you receive a completion certificate. Keep this — you will need it when you sell.
Planning to finance your loft conversion through a remortgage? Use our Mortgage Calculator on CalcPad to see what releasing equity would cost you per month.
Last updated May 2026. Costs are estimates and vary by location, property type, and specification. Always get multiple quotes from specialist loft conversion companies.