By Sarah Macklin 
Written by Andy at TS Builders
If you’re reading this, you’ve probably already Googled “loft conversion cost”, and you’ve probably seen numbers like £25,000 to £45,000.
Here’s the problem: those are national averages. They include conversions in Sunderland, Stoke-on-Trent, and Swansea. If you live in London, and particularly in a Victorian or Edwardian terrace, those numbers are about as useful as a chocolate teapot.
I run TS Builders in South West London. Since 2009, we’ve completed over 500 loft conversions across Clapham, Wandsworth, Wimbledon, and Putney. Here’s what the numbers actually look like in 2026.
The real numbers for a London loft converson
For a standard London terrace or semi-detached, this is what you’re looking at:
Rooflight (Velux) Conversion — £28,000-£37,000
The simplest option. You’re keeping the existing roof structure and adding rooflights. No dormers, no major structural changes. Works well if you already have decent head height. 6-8 weeks.
Rear Dormer — £40,000-£52,000
The most common choice for Victorian terraces. A flat-roofed box extends across the rear slope of the roof, creating a full-width room with proper head height. 8-10 weeks.
Hip-to-Gable — £46,000-£60,000
For semi-detached or end-of-terrace properties where the roof slopes in from the side. You’re extending the sloping (hipped) side vertically to create a full gable wall. 8-12 weeks.
Mansard — £55,000-£72,000
The big one. The entire roof is replaced with a steep-sided structure, typically with dormer windows. Maximum space, maximum cost. Often requires planning permission. 10-14 weeks.
These prices include everything: materials, labour, scaffolding, skips, building control fees, and VAT. If a quote comes in significantly lower, ask what’s been left out — electrics, plumbing, and plastering are the usual suspects.
The 3 costs no one warns you about
1. Party Wall Surveyors — £1,500-£2,500 per neighbour
The Party Wall Act 1996 says your neighbour can’t unreasonably withhold consent. But they are entitled to a surveyor, and you pay for it. If your neighbour appoints their own surveyor and you don’t have an agreed surveyor acting for both sides, that’s £1,500-£2,500 per neighbour. On a mid-terrace with two adjoining walls, you could be looking at £5,000 before a single brick is moved.
Serve party wall notices early. The Act requires two months’ notice. Start the conversation with your neighbour before the formal notice lands on their doormat.
2. Structural Steel — £2,000-£5,000
Many Victorian lofts were never designed to carry a floor, just a ceiling and a roof. When you strip out the old ceiling joists and install a new floor structure, you often discover the party wall isn’t strong enough to take the additional load. Steel beams to spread the weight: £2,000-£5,000. Your architect should identify this at the design stage. Not all of them do. Ask yours specifically: “Have you checked whether the party wall needs reinforcing?”
3. Asbestos (Still, in 2026)
If your house was built before 2000 and has textured ceiling coatings, there’s roughly a 30% chance they contain asbestos. Testing costs about £80. Removal adds £800-£1,500. The alternative of not testing is not a strategy worth considering.
Planning permission vs. permitted development
Most loft conversions fall under Permitted Development (PD), meaning you don’t need full planning permission. But there are limits:
– The additional volume must not exceed 40m³ for terraced houses or 50m³ for detached and semi-detached
– The extension cannot extend beyond the existing roof slope on the principal elevation (the front)
– Materials must be similar in appearance to the existing house
– No balconies, verandas, or raised platforms
If you’re in a conservation area, PD rights may be restricted. If your property is listed, you’ll need Listed Building Consent regardless.
Wandsworth, Lambeth, Merton, and Richmond each have slightly different interpretations of PD rules — particularly around dormer size and roof materials. A builder who knows your specific borough’s planning department will save you months of back-and-forth.
What is the timeline for a loft conversion?
A builder will tell you the build takes 8-10 weeks. That’s true for the construction phase. But the whole process looks more like this:
– Architectural drawings: 3-5 weeks
– Planning permission (if required): 8-12 weeks
– Party wall notices: 2 months minimum
– Build: 8-10 weeks
– Snagging and sign-off: 1-2 weeks
Real total: 5-7 months from decision day to moving the furniture in. If you call a builder in January, you won’t have a finished loft before summer.
The 4 things to get right before you start
Get three quotes, not two. The spread between the highest and lowest quote for the same dormer loft in SW London is routinely £15,000 or more.
Ask: “Is this a finished price including everything?” Some builders quote a “shell”: structure, roof, and windows. No electrics, no plumbing, no plastering, no decorating. That’s not a loft conversion. Get the finished price in writing.
If there’s no written guarantee, walk away. We provide a 5-year workmanship guarantee on every project. If a builder dodges this question or says “don’t worry about that,” they’re telling you how they’ll handle things when something goes wrong.
Don’t start in November. London winters are wet, materials swell, scaffolding is slower to work on, and everything takes 2-3 extra weeks between November and February. Wait until March if you can.
The question nobody asks but definitely should
“How many loft conversions have you done in my postcode?”
Different boroughs have different planning policies. Different streets have different planning histories. A builder who’s completed 50 lofts in Wandsworth knows what Wandsworth Council’s planning officer will accept. A builder who’s done 50 lofts in Birmingham has no idea. Local experience isn’t a bonus — it’s the difference between your application being approved in eight weeks or refused at week ten.
We’ve completed over 500 projects across SW London since 2009. We know Wandsworth’s quirks, Lambeth’s conservation area boundaries, and Merton’s building control preferences.
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Andy runs TS Builders, a South West London construction company specialising in loft conversions, house extensions, and full home renovations. Fixed-price contracts with a 5-year workmanship guarantee. Visit tsbuilders.co.uk for a free quote.
