House Extension
In plain English: Adding floor area to a UK home by building out or up. Single-storey rear, side-return, double-storey and wraparound are the four main types. Industry guidance commonly cites 10–25% property-value uplift depending on the type and whether a bedroom is added.
A house extension adds floor area to a UK home by building outward or upward. The four main UK types are single-storey rear, side-return, double-storey (side or rear) and wraparound. Each has a different planning and cost profile, and each adds a different amount to property value.
The four main UK extension types
Single-storey rear extension (15–25 m² typical). The default UK extension — bigger kitchen-diner, often a utility room. Typical UK 2026 cost £45–80k; typical uplift 10–15% of property value.
Side-return extension (8–15 m² typical). Fills the unused side passage on Victorian and Edwardian terraces. Among the highest £-per-£ returning UK extensions. Typical UK 2026 cost £35–65k; typical uplift 10–20%.
Double-storey side or rear extension (30–60 m² total typical). Adds floor area on two storeys — typically bigger kitchen down and a bedroom + ensuite up. Highest absolute uplift. Typical UK 2026 cost £70–140k+; typical uplift 15–25%.
Wraparound extension (rear + side return combined, 20–40 m² typical). Maximises the ground-floor footprint. Typical UK 2026 cost £80–150k; typical uplift 15–22%.
Cost figures are 2026 indicative ranges. Use a Federation of Master Builders member or a quantity surveyor for a specific quote.
Why double-storey usually beats single-storey in % terms
A single-storey extension adds floor area but typically not a bedroom — moving incremental value but not crossing a price band. A double-storey extension adds floor area and a bedroom — capturing both incremental floor-area value and the price-band jump from going to 4-bed (or 5-bed) territory.
Where Offrly fits
Use Offrly's free property valuation and the Scenario Explorer to model the uplift on your specific home. Drag the floor-area slider up by the extension's added area (e.g. +20 m² for a typical rear extension); for double-storey extensions, drag the bedroom and bathroom sliders up too. The Recalculate delta is the modelled uplift against the same live UK comparable set.
Read the full extension value guide for the 2026 picture.
Indicative market guidance — not a regulated valuation and not financial, tax or legal advice. For mortgage, insurance, probate or tax purposes, consult a RICS-qualified surveyor and an independent qualified adviser. For build cost estimates use a Federation of Master Builders member or a quantity surveyor.
Related terms
- Planning permission — many single-storey rear extensions fall under permitted development
- Party-wall — required for any work on the party wall in a semi or terrace
- Loft conversion — typically a better £-per-£ uplift than a single-storey extension
- Building survey — structural inspection often needed before signing off
FAQ: House Extension
How much value does an extension add to a UK house?
Industry guidance commonly cites 10–15% uplift for a single-storey rear extension and 15–25% for a successful double-storey extension that adds a bedroom upstairs. Side-return extensions in high-£/sqft markets (London Victorian terraces) commonly add 10–20%. The actual number for a specific home depends on the postcode's price per square foot and whether the extension adds a bedroom — model the specific uplift with Offrly's free Scenario Explorer at /value.
Does an extension need planning permission?
Single-storey rear extensions often fall under permitted development in England, up to 3 m from the original rear wall on semis/terraces and 4 m on detached, with prior approval extending these to 6 m and 8 m respectively. Side-return and double-storey extensions almost always need full planning permission. Conservation areas, listed buildings and Article 4 directions remove permitted development. Building regulations approval is required for every extension.
Is an extension or a loft conversion better for adding value?
On most semi-detached and terraced UK homes with the loft headroom, a loft conversion outperforms a single-storey rear extension in pure percentage uplift terms because it adds a bedroom without consuming garden space. A double-storey extension that also adds a bedroom often beats a loft for absolute pounds, but at higher cost. The best lever depends on the specific house — model both with Offrly's Scenario Explorer.