How Much Does a Loft Conversion Cost in London? (2026 Guide)

Everyone in London looks up at their roof eventually and thinks "there's a spare bedroom hiding up there". And you're right – it's usually the smartest way to add value to a property in the capital. But look, I've seen quotes vary by £20k for the exact same job on the exact same street. Why? Because some builders quote you for the shell, and others quote for a finish where you can actually live.

I'm going to tell you what it really costs to get the job done properly in 2026—steels, insulation, stairs and all. No sales talk, just the numbers.

Average Loft Conversion Costs in London (2026 Data)

London prices are a different beast to the rest of the UK. We have parking suspensions, tight terraced streets, and higher labour rates. Based on projects we've priced and built across Greater London this year, here are the realistic brackets you should be budgeting for.

Conversion Type	Estimated Cost	Timeline
Velux (Roof Light)	£30,000 - £45,000	4-6 Weeks
Rear Dormer	£45,000 - £65,000	6-8 Weeks
Hip-to-Gable	£55,000 - £75,000	7-9 Weeks
L-Shaped Mansard	£70,000+	10-12 Weeks

💡 Builder's Truth: Websites might tell you £25k. In London? Impossible unless you're doing the labour yourself. If a quote looks too cheap, check if they've included the bathroom suite, flooring, and decorating. Usually, they haven't.

1. Velux / Skylight Conversion

This is the most cost-effective option. We don't alter the roofline; we just reinforce the floor, insulate the roof, and put windows in. Ideal if you have plenty of headroom (2.2m+ at the highest point).

2. Rear Dormer (The London Classic)

Walk down any street in Clapham or Walthamstow and you'll see these. We extend vertically from the rear roof slope. It creates a proper square room with full headroom. By far the best value for money in terms of space gained.

The "Hidden" Costs Layouts Often Miss

When you're budgeting, don't just look at the build cost. You strictly need to factor these in:

Party Wall Agreements: In London terraces, you need agreement from neighbours on both sides. Budget £1,000 per neighbour if they dissent and want a surveyor.
Moving the Water Tank: Old London houses often have the cold water tank in the loft. That needs to go. We usually switch you to a combi boiler or a megaflow system.
Architect & Structural Engineer: You can't just guess the steel sizes. Budget £1,500 - £2,500 for proper drawings and calculations.

Planning Permission vs. Permitted Development

Good news: Most loft conversions fall under Permitted Development (PD) rights. This means you don't need full planning permission as long as:

You add less than 40 cubic metres (terraced) or 50 cubic metres (semi/detached).
The dormer doesn't go higher than the existing ridge.
You use materials that match the existing house.

However, if you live in a Conservation Area or a flat (not a house), PD rights don't apply. You strictly need full planning.

Not sure if your loft counts as Permitted Development?

It can be tricky with hip-to-gable calculations. Don't guess.

📸 Send me a photo on WhatsApp

I'll usually be able to tell you in 5 minutes.

Is it Worth the Investment?

In short: Yes. A quality loft conversion adds roughly 20% to the value of a London property. If your house is worth £600k, that's £120k value added for a £50k spend. Plus, you save the £30k+ cost of stamp duty and moving fees if you tried to upsize instead.

But remember what I said at the start—cut corners on the insulation or the finish, and you won't get that value back. Do it right, or don't do it.