The short answer
A side return extension is usually the cheaper, brighter and less disruptive choice for terraced and semi-detached homes, while a basement adds far more total space but at a much higher cost. A side return fills the narrow alley beside a kitchen, often costing around £2,000–£3,000+ per square metre, and is popular because it widens the ground floor and floods it with light through a glazed roof. A basement adds a whole new storey beneath the house but, as a dig-out, runs to £3,000–£5,000+ per square metre with major structural work. The side return wins for most homes wanting a brighter, wider kitchen-diner; the basement makes sense where you need much more space and the side return alone is not enough.
The side return extension and the basement are both common ways to add space to a terrace, but they solve different problems. The sections below compare them on cost, light, space and disruption to help you choose.
At a glance
- Side return cost~£2,000–£3,000+/m²
- Basement (dig-out)~£3,000–£5,000+/m²
- Adds most spaceBasement
- Brightest resultSide return (glazed roof)
- Less disruptiveSide return
What each one adds
A side return extension uses the narrow strip of land that runs alongside the back of many Victorian and Edwardian terraces, the gap beside the original rear kitchen. Filling it in widens the ground floor, typically creating an open-plan kitchen-diner that spans the full width of the house. With a glazed roof or large rooflights, the result is bright and connected to the garden, which is why side returns are so popular for transforming cramped galley kitchens.
A basement adds a completely new storey beneath the house, far more floor area than a side return, suitable for bedrooms, a family room, a gym, an office or an annexe. It does not widen the ground floor or improve the kitchen; it adds space below. So the two are not really like-for-like: the side return reshapes and brightens the existing ground floor, while the basement adds a separate level of accommodation.
Cost, light and disruption
On cost, the side return is usually the more economical per square metre and adds less total area, so the overall spend is typically lower than a basement dig-out. It also delivers excellent natural light through its glazed roof, a key part of its appeal, whereas a basement must engineer light in through a lightwell. On disruption, a side return is a contained single-storey build of a few months, while a basement dig-out is a major structural project with underpinning and spoil removal, often lasting longer and requiring the household to adapt around it.
Both may need planning permission and, in a terrace, a party-wall agreement with neighbours. The basement's structural complexity makes its consents and design more involved. The comparison below summarises the differences.
| Factor | Side return extension | Basement |
|---|---|---|
| Cost/m² | £2,000–£3,000+ | £3,000–£5,000+ |
| Total space added | Moderate | Large (full storey) |
| Natural light | Excellent (glazed roof) | Engineered (lightwell) |
| Disruption | Moderate | High |
| Best for | Brighter, wider kitchen | Much more space |
Indicative comparison for guidance. Costs and value depend on the property and local market.
How to choose
Decide by what your home actually lacks. If your main frustration is a narrow, dark kitchen and a cramped ground floor, the side return is almost always the better answer: it directly fixes that problem, adds light, and does so at a lower cost and with less disruption than a basement. It is the classic improvement for a period terrace and tends to add good value because a bright, open kitchen-diner is widely wanted by buyers.
If your need is for significantly more space, additional bedrooms, a family room, an office or an annexe, that a side return cannot provide, the basement is the route that delivers a full extra storey, provided the cost is justified by local prices and your need for the space. In high-value urban areas the basement's higher spend is more easily recovered. For many homes the practical path is the side return first, because it gives the biggest everyday improvement for the money, with a basement reserved for when the ground floor is already optimised and the household still needs more room. Consider also combining a side return with a wider rear extension before committing to the far larger investment of digging down.
Frequently asked questions
Is a side return extension cheaper than a basement?
Usually yes. A side return at around £2,000–£3,000 per square metre adds less total area than a basement and avoids excavation and underpinning, so the overall spend is typically lower than a basement dig-out at £3,000–£5,000 or more.
Which adds more space, a side return or a basement?
A basement adds far more, as it creates a whole new storey beneath the house. A side return fills the narrow alley beside the kitchen to widen and brighten the ground floor, adding a more modest amount of space.
Can I do both a side return and a basement?
Yes, and some owners do, though the combined cost and disruption are significant. Many start with a side return for a brighter kitchen-diner and only add a basement later if they need substantially more space.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.