L-shaped Cost Calculator
An L-shaped loft conversion in 2026 UK costs £58,000–£95,000 all-in for a typical Victorian/Edwardian terrace or semi with rear addition. London and the South East push the upper end to £110,000+; the North and Midlands often complete around…
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2 steps, using the figures from our cost guide.
How this calculator works
Every figure in this calculator is the one published in the cost table on our l-shaped guide. Pick the option that matches your job and the calculator returns that published range, adjusted for your region - it does not model anything the guide does not already say.
That is deliberate. A calculator whose rates drift from the page it sits under is worse than no calculator, because two numbers on the same site disagree.
What it cannot know: access, the condition of what is already there, and anything structural. Those are what separate three real quotes from each other.
Published l-shaped costs (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| North East / Yorkshire | £52,000–£68,000 | |
| North West / Midlands | £56,000–£75,000 | |
| South West / East | £65,000–£85,000 | |
| Outer London | £72,000–£95,000 |
These are the figures from our l-shaped cost guide.
What drives an L-shaped loft conversion’s cost
An L-shaped conversion costs more than a plain dormer because it is really two connected conversions: the main dormer over the original roof plus a second dormer over the rear addition, meeting in an L. That means more roof structure, more steelwork to tie the two together, a larger floor area and a longer build, which is why the published bands run from about £52,000 in the North to £95,000 or more in and around London. The layout only works on houses that already have a back addition — typically Victorian and Edwardian terraces and semis — so the starting roof is often the first thing a surveyor prices.
Within that, the parts that move a quote most are the steel beams needed to open up the two roof sections, where the staircase can land without eating a bedroom below, and head height — if the ridge is low, a change to the roofline gets expensive fast. Adding an en-suite or bathroom brings plumbing and waste runs into the roof, and party wall agreements are almost always needed on terraces and semis because you are working right up to a shared wall. Access for materials and a scaffold around the back of the house is the last big variable.
Two things are worth reading before you commit: our loft conversion cost guide sets the L-shaped price against dormer, hip-to-gable and mansard options, and the planning permission for a loft conversion guide explains why L-shaped jobs more often need a full application than a simple rear dormer. If you are at the earliest stage, the step-by-step planning guide sets out the order to tackle it in.
L-shaped calculator — FAQs
It uses the same published figures as the cost guide it sits under, adjusted for your region. It is a budgeting estimate, not a quote - only a site visit produces one of those.
Because it is effectively two conversions in one: a dormer over the main roof plus a second dormer over the rear addition, joined in an L. That adds roof structure, extra steelwork to tie the sections together, more floor area and a longer build than a single rear dormer, which is why the typical range sits around £52,000–£95,000 depending on region rather than the lower figures you see for a plain dormer.
More often than a simple dormer. Because an L-shaped conversion is larger, it can exceed the volume allowance for permitted development, and many of the terraces and semis it suits sit in conservation areas where permitted development is restricted — both of which mean a full application. You will also almost certainly need a party wall agreement with attached neighbours. Confirm the position with your local planning authority before design work goes far.
Related cost guides
- L-shaped cost guide — The full price guide
- All loft conversions services — Browse the hub
- All BestBuilders services — Every trade we cover
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