A loft conversion is usually the most cost-effective way to add a bedroom and real value to your home without losing any garden. But prices vary a lot, and the type of conversion you choose makes the biggest difference. This guide explains what a loft conversion typically costs in 2026, what drives the price up or down, and the extra fees most homeowners forget to budget for.
The figures below are typical ranges for Slough, Berkshire and the wider South East, based on published 2026 UK price guides. Every loft is different, so treat them as a starting point. For an exact figure, book a free survey and fixed-price quote.
Loft conversion costs at a glance (2026)
| Type of conversion | Typical cost (South East) | Best for |
|---|---|---|
| Velux / rooflight | £25,000 – £40,000 | Lofts that already have good head height |
| Rear dormer | £45,000 – £65,000 | Most terraced and semi-detached houses |
| Hip-to-gable (with rear dormer) | £50,000 – £70,000 | Semi-detached and detached houses with a sloping side roof |
| Mansard | £60,000 – £85,000+ | Maximum space, often terraced houses |
Prices include the build, stairs, insulation, electrics, plastering and decoration to a standard finish. London prices are usually higher, and a high-spec en-suite or bespoke joinery will push any project towards the top of its range.
Costs by type of loft conversion
Velux (rooflight) loft conversion: £25,000 – £40,000
The simplest and cheapest option. The roof shape stays the same and roof windows are added to bring in light. It only works if your loft already has enough head height (around 2.2 metres at the highest point is a good rule of thumb). It rarely needs planning permission, and the work is quick because there are few structural changes to the roof.
Dormer loft conversion: £45,000 – £65,000
The most popular choice in the UK. A box-shaped extension is built out from the rear roof slope, creating full head height across most of the room. A rear dormer can usually fit a double bedroom and an en-suite, and on most houses it can be built under permitted development, without a full planning application.
Hip-to-gable loft conversion: £50,000 – £70,000
If your house has a sloping (hipped) side roof, as many semi-detached and detached homes in Slough and Berkshire do, the side slope is built up into a vertical gable wall. This is usually combined with a rear dormer, which is why it costs more. It creates a much larger, squarer room and space for the staircase to land comfortably.
Mansard loft conversion: £60,000 – £85,000+
A mansard rebuilds the rear roof slope at a near-vertical angle, with a flat roof on top. It gives the most space of any loft conversion, but it involves the most structural work and almost always needs planning permission. That makes it the most expensive and slowest option.
How much does a loft conversion cost on a 3-bed semi?
A 3-bed semi is the most common home we're asked about. For a typical 3-bed semi in Slough or Berkshire, expect roughly:
- Rear dormer: £45,000 – £60,000 for a bedroom and en-suite.
- Hip-to-gable with rear dormer: £55,000 – £70,000. It costs more, but it gives a noticeably bigger room.
The final price depends on the size of the loft, where the staircase can go, and the finish you choose.
Extra costs to budget for
The build price is only part of the total. Make sure your budget covers these as well (typical figures, which can vary):
- Architectural drawings: around £1,500 – £3,500.
- Structural engineer's calculations: around £500 – £1,500. They're needed for the new floor joists and steel beams.
- Building control: every loft conversion needs Building Regulations approval. Fees vary by council and project size, often several hundred pounds to £1,500+.
- Planning: a Lawful Development Certificate (to prove the work is permitted development) costs £264. A full householder planning application in England costs £528.
- Party wall agreement: if you share a wall with a neighbour (terraced and semi-detached homes), you'll usually need to serve notice under the Party Wall Act. If your neighbour appoints a surveyor, budget roughly £1,000 – £1,500 per neighbour.
- Fit-out choices: en-suite bathroom, built-in wardrobes, flooring and window upgrades can add several thousand pounds.
What affects the price?
- Type and size of conversion: the biggest factor by far.
- Staircase position: fitting a new staircase may mean reconfiguring the landing or a bedroom below.
- Existing roof structure: older cut roofs and modern trussed roofs need different structural work.
- En-suite: adding a bathroom means extra plumbing, drainage and finishes.
- Fire safety: Building Regulations require a protected escape route. This can mean fire doors, and sometimes a mains-wired alarm system or sprinklers.
- Access and scaffolding: most lofts are built from scaffolding, so the cost depends on the house and the street.
- Planning status: homes in a conservation area, or flats and maisonettes, will usually need full planning permission.
Do you need planning permission?
Many Velux and rear dormer conversions are permitted development. That means no full planning application is needed, as long as the extra roof space stays within 40 cubic metres for terraced houses or 50 cubic metres for semi-detached and detached houses, and other conditions are met. Hip-to-gable conversions can often be done this way too. Mansards and anything facing the street usually need planning permission. Whatever the route, every loft conversion needs Building Regulations approval.
For a local view of the rules, read our guide to planning permission in Slough for extensions and loft conversions, and our guide to loft conversion building regulations.
Is a loft conversion worth it?
For most families, yes. A loft conversion usually costs less per square metre than moving house once you add up stamp duty, agents' fees and removals. A well-designed extra bedroom with an en-suite also tends to add significant value. It's also one of the least disruptive projects: most of the work happens from scaffolding, so family life can carry on downstairs.
Get an exact price for your loft
Online guides can only give you a range. We'll survey your loft, tell you which type of conversion suits your house, and give you a clear, fixed-price quote, with design, approvals and the build all managed by one team.
We convert lofts across Slough, Reading, Bracknell, Berkshire and London. See our loft conversion service or call 07969 304472 to book a free site visit.
Loft conversion cost: FAQs
What is the cheapest type of loft conversion?
A Velux (rooflight) conversion. The roof shape doesn't change, so there is far less structural work. It only suits lofts that already have enough head height.
How long does a loft conversion take?
Most dormer and hip-to-gable conversions take around 6 to 10 weeks on site. Allow extra time beforehand for drawings, approvals and any party wall notices.
Does a loft conversion add value?
Usually, yes. Adding a bedroom, especially with an en-suite, is one of the most effective ways to increase the value of a typical family home.
Do I need planning permission for a dormer loft conversion?
Often not. Most rear dormers fall under permitted development if they stay within the volume limits and conditions. They do still need Building Regulations approval. Homes in conservation areas usually need full planning permission.
Get a free site visit and a fixed-price quote.
