The short answer
A rear extension is usually the better choice when you have garden space to build into, while a basement makes more sense on a constrained plot where you cannot extend outwards. Extensions are generally cheaper per square metre, often £2,000–£3,000+, get natural light and garden access easily, and are less disruptive. Basements cost more, especially a dig-out at £3,000–£5,000+ per square metre, and need engineered light and waterproofing, but they add space without losing any garden. In high-value urban homes with little outdoor space, a basement can add more total value; where there is garden to spare, a rear extension typically gives better space and return for the money. The right answer depends on your plot, light, budget and local prices.
Both a basement and a rear extension add ground-level living space, but they suit very different plots and budgets. The sections below compare them on cost, light, garden impact and value to help you choose.
At a glance
- Rear extension cost~£2,000–£3,000+/m²
- Basement (dig-out)~£3,000–£5,000+/m²
- Keeps gardenBasement
- Easier light & accessExtension
- Best on tight plotsBasement
Cost, light and garden trade-offs
The core trade-off is straightforward. A rear extension builds outwards into the garden, which is usually cheaper and simpler than excavating downwards, gives easy natural light through windows and doors, and connects directly to the outdoor space. Its cost is the loss of some garden, which on a small plot can be a real drawback. A basement builds downwards, so it keeps the garden intact and adds space where there is no room to extend, but it costs more, needs a designed lighting strategy and full waterproofing, and is a more demanding build.
For many homes with a reasonable garden, the extension is the more efficient way to add bright, well-connected space. For homes with little or no garden, or where the garden is too precious to lose, the basement becomes the logical route despite its higher cost.
| Factor | Rear extension | Basement |
|---|---|---|
| Typical cost/m² | £2,000–£3,000+ | £3,000–£5,000+ |
| Natural light | Easy | Engineered (lightwell) |
| Garden impact | Reduces garden | Keeps garden |
| Disruption | Moderate | Higher (dig-out) |
| Best where | Garden to spare | Constrained plot |
Indicative comparison for guidance. Costs and value depend on the property and local market.
Disruption, planning and value
The two projects differ in how they affect the household and what consents they need. A rear extension is a moderate build; smaller single-storey extensions may fall under permitted development, while larger ones need planning permission, and a party-wall agreement may apply in a terrace. A basement dig-out is a major structural project with months of disruption, spoil removal and underpinning, almost always needing planning permission, structural design and a party-wall agreement where it shares a wall.
On value, both add usable floor area, so the uplift again tracks local prices. An extension that creates a bright kitchen-diner opening onto the garden is a strong, widely wanted feature and usually adds good value at a sensible cost. A basement adds value too, but the higher cost means it only out-performs the extension where the local rate per square metre is high, or where there is simply no garden to extend into and the extra space is otherwise unobtainable. In those constrained, high-value situations, the basement's ability to add space without sacrificing the garden is exactly what justifies it.
How to choose
Start with the plot. If you have a garden you can comfortably build into without losing space you value, a rear extension is usually the better-value, brighter and less disruptive option, and is the right first choice for most homes. If your garden is small, north-facing and precious, or if there is effectively no outdoor space to extend into, the basement comes into its own by adding a full room without reducing the garden at all.
Then consider the local market: in high-value urban areas the basement's higher cost is more easily recovered, while in average areas the extension's efficiency usually wins on return. Think about what the space is for, since an extension naturally suits a garden-connected kitchen or living room, whereas a basement suits a family room, gym, office or annexe that does not need a garden outlook. Finally, weigh the disruption and consents you are willing to take on, as the extension is the gentler route and the dig-out the bigger commitment. For most homes with a garden, the extension is better value; the basement is the right answer when the plot leaves no other way to add the space and the location supports the spend.
Frequently asked questions
Is a basement or rear extension cheaper?
A rear extension is usually cheaper per square metre, often £2,000–£3,000 or more, while a basement dig-out commonly runs £3,000–£5,000 or more. Extensions build outwards into existing space, whereas basements require excavation and waterproofing.
When is a basement better than an extension?
A basement is better on a constrained plot with little or no garden to build into, or where the garden is too valuable to lose. In high-value urban areas it can also add more total value despite the higher cost.
Which gets more natural light?
A rear extension gets natural light easily through windows and doors and connects to the garden. A basement needs light engineered in through a lightwell, sunken courtyard or rear glazing, which adds cost and design work.
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.