Which adds more value — extension or loft conversion?

Roughly similar in absolute value uplift on a typical London property — both in the £85,000–£160,000 range on a £600k–£1m house. The loft typically wins slightly on ROI percentage when it creates a bedroom bracket change (3-bed to 4-bed). The extension wins when it transforms a closed-plan ground floor into the open-plan layout expected by modern buyers.

Can I do both at the same time?

Yes — many London projects combine an extension and a loft conversion into a single build. Saves on professional fees, party wall (single Award covers both), planning (single application) and reduces total disruption. Build time 18–26 weeks total. Cost £140,000–£260,000 combined. Cash flow is the main constraint.

Which is faster to build?

A loft conversion is typically faster — 8–14 weeks on site vs 10–18 weeks for an extension. Pre-construction is similar for both (6–12 weeks for design, building regs, party wall). Lofts are faster because most work happens above the existing roof line, with less groundworks. Extensions involve foundation excavation, drainage diversion and structural work to the existing rear wall.

What if my budget only covers one?

Pick by use case. Need more bedrooms or a bigger master suite — loft conversion. Need a bigger family kitchen-diner or to fix a cramped ground floor — extension. Both add similar absolute value, so this is a use-case decision more than a financial one. Builderr's design consultation includes a free comparison for your specific property.

Extension vs Loft Conversion — London 2025 Comparison

Extension vs Loft Conversion — Which Is Better?

An extension adds ground-floor living space (kitchen-diner, family room) and typically costs £65,000–£140,000 in London. A loft conversion adds bedrooms upstairs and costs £55,000–£140,000. Choose an extension for more entertaining and family living space; choose a loft for more bedrooms and a higher bracket buyer. Many London homes benefit from doing both — a kitchen extension first, loft conversion second, over 2–4 years.

What each does for your home

An extension (typically a rear or side return) adds ground-floor living space. The standard London use case is a kitchen extension — opening up the dated galley kitchen into an open-plan kitchen-diner that connects to the garden via bifold doors. This transforms how the family uses the ground floor: cooking, dining and casual living happen in one connected space. A loft conversion adds bedrooms and bathrooms upstairs. The standard London use case is a dormer creating a master suite with ensuite, allowing the existing first-floor bedroom layout to be reconfigured (e.g. converting the previous master to a kids' room). The fundamental question: do you need more living space or more bedrooms?

Cost comparison

Extension costs in 2025 London (zones 2–4): side return £55,000–£85,000; 4m rear £90,000–£125,000; 6m rear under prior approval £105,000–£145,000; wraparound £120,000–£180,000; double-storey rear £155,000–£235,000. Loft conversion costs (zones 2–4): Velux £35,000–£55,000; small dormer £55,000–£68,000; standard dormer with ensuite £65,000–£85,000; L-shape £85,000–£115,000; mansard £95,000–£140,000. Both have a similar entry point (£55,000) but extensions tend higher at the top end because of the kitchen fit-out cost and larger glazing. Per square metre, lofts can be slightly cheaper than extensions in some configurations.

Value uplift and ROI

On a £600,000–£1m London property (zones 2–4), an extension typically adds £80,000–£160,000 (10–18% uplift) — the open-plan kitchen-diner is now an expected feature in this price bracket. A loft conversion typically adds £85,000–£160,000 (15–22% uplift) — the value comes from the bedroom bracket change (3-bed becoming 4-bed). ROI ratio (value added vs cost) is similar for both — typically £1.30–£1.70 returned per £1 spent for well-executed projects. The biggest ROI difference is when one of them changes a buyer bracket: a kitchen extension that creates an open-plan space in a previously closed-plan house is transformative; a loft that takes a 3-bed to a 4-bed similarly transformative.

Which to do first if you want both

Many London homeowners want both an extension and a loft conversion eventually. The standard sequencing is extension first, loft second, because: (1) the kitchen extension is typically the more disruptive build (groundworks, drainage diversion, ground-floor reconfiguration) and you want this done while the house is more flexible; (2) the kitchen-diner becomes the family's daytime hub for the duration of the loft build; (3) value uplift is realised sooner — the kitchen extension's value is immediate, while the loft's bedroom uplift compounds the property value when sold. Some homeowners do both simultaneously — saves on professional fees and disruption, but cash flow is significantly higher (£140,000–£260,000 total).