Extension vs Loft Conversion: Which Is Cheaper? (2026)
Both add space and value, but on price they rarely tie. A rooflight (Velux) loft conversion typically starts around £25,000, while a single-storey rear extension usually begins near £40,000 and a dormer loft or two-storey extension costs more again. The cheaper option depends on how much space you need, your roof height, your garden and planning — here is the side-by-side.
Extension vs loft conversion — the short answer
- Usually cheaper: a rooflight loft conversion, from around £25,000, reuses the existing roof space and footprint.
- More expensive: a single-storey extension from ~£40,000, or a dormer loft / two-storey extension above that.
- Best for an extra bedroom: a loft conversion — it keeps your garden and is often permitted development.
- Best for a bigger kitchen or living space: a ground-floor extension.
- Check first: loft needs ~2.2m ridge height; an extension needs garden space and foundations.
If your only goal is one more bedroom, a loft conversion is almost always the cheaper route because it uses space you already own under the roof. If you need to enlarge the kitchen, dining or living area, only an extension delivers that ground-floor footprint — so the “cheaper” option really depends on what space you are trying to create.
Extension vs Loft Conversion — Compared (UK 2026)
Indicative fitted costs and what each option adds. Actual prices depend on size, spec, access and location (London and the South East run higher).
Figures include the build but exclude fit-out extras such as high-end kitchens or bathrooms. A loft conversion also needs a compliant staircase, which uses some space on the floor below.
Which Option Is Right for You?
Choose a loft conversion if…
You want the cheapest way to add a bedroom or home office, you want to keep your garden, and your roof has enough head height. Many fall under permitted development.
Choose an extension if…
You need a bigger kitchen, dining or living area at ground-floor level, you have garden space to give up, and open-plan living matters more to you than a spare bedroom.
Think about added value
Both can add value, but a loft bedroom with an en-suite and a bright kitchen extension are among the highest-return projects — pick the one that fixes what your home most lacks.
Check feasibility first
A loft needs roughly 2.2m of ridge height and room for a staircase; an extension needs garden space, foundations and good access. A surveyor or builder can confirm before you commit.
Common Questions
Related Guides
More compare, cost and how-to guides for your extension or loft project.
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