Almost every basement conversion guide quotes the same statistic: a 10 to 20% uplift in property value. That figure is directionally true and almost useless on its own, because 10 to 20% of very different starting values produces very different cash outcomes, and it says nothing about whether a specific project actually clears its own cost.
This guide grounds that percentage in real numbers, using four London boroughs already covered across our blog: Fulham, Hampstead, Wandsworth and Wimbledon. It pairs naturally with our basement waterproofing guide, since a poorly specified waterproofing system is the single fastest way to erase any value a basement conversion would otherwise add.
Bottom line
Basement conversions typically add 10–20% to property value in London, but the cash outcome depends entirely on the borough's baseline property values. On a lower-value terrace the uplift can sit close to build cost; on a higher-value property in a space-constrained borough, the same percentage produces a substantially larger cash return.
Why a National Percentage Does Not Tell You Much
A 10 to 20% uplift on a £400,000 property and a 10 to 20% uplift on a £3,000,000 property are not comparable outcomes in cash terms, even though both are accurately described by the same statistic. Build cost, by contrast, does not scale anywhere near as steeply with property value; a basement conversion costs broadly the same to excavate, waterproof and fit out regardless of what the finished house is likely to sell for. This is why the borough a property sits in matters more to the value case than almost any other single factor.
Worked Example: Fulham
A typical Victorian terrace in Fulham sits in the region of £1.2m to £1.4m. A basement conversion in this size of property typically costs £120,000 to £220,000 depending on scope, with a 10–20% value uplift translating to roughly £130,000 to £260,000. On the numbers alone, a well-executed Fulham basement conversion sits close to cost-neutral to modestly positive, before accounting for the additional living space itself, which is the primary reason most clients commission one.
Worked Example: Hampstead
Hampstead's larger detached and semi-detached properties typically sit in the £2.5m to £3.5m range. A basement conversion of a similar scope, £150,000 to £300,000, against a 10–20% uplift of roughly £300,000 to £600,000, produces a materially stronger cash case than the Fulham example above, purely because the baseline property value is higher. This is consistently where we see the strongest financial case for a basement conversion across the boroughs we work in most.
Worked Example: Wandsworth
Wandsworth terraces typically sit around £1.1m to £1.3m, close to the Fulham figures above. A comparable basement conversion at £100,000 to £200,000 against a 10–20% uplift of roughly £120,000 to £240,000 again produces a broadly cost-neutral to positive outcome, reinforcing that the two boroughs behave similarly on this specific calculation despite being on opposite sides of the river.
Worked Example: Wimbledon
Wimbledon's larger family homes typically sit around £1.7m to £2.1m. A basement conversion at £130,000 to £250,000 against a 10–20% uplift of roughly £190,000 to £380,000 sits between the Fulham and Hampstead examples, consistent with Wimbledon's property values sitting between the two.
What Actually Drives the Uplift Beyond Square Footage
The percentage itself is not fixed; it moves with the quality of what is delivered. Genuine natural light via a front or rear lightwell, ceiling height above the bare minimum required for the room to feel habitable rather than a converted void, and a considered use for the space, a guest suite or home cinema rather than generic storage, all push a basement conversion toward the upper end of the value range rather than the lower. A basement that reads as an integrated part of the house, rather than a converted cellar, is what buyers and valuers are actually responding to.
The Cost-to-Value Question: When Does It Make Sense?
Where a basement conversion sits close to cost-neutral on paper, as in the Fulham and Wandsworth examples above, the case for proceeding usually rests on the value of the additional living space to the household now, not purely on resale arithmetic. Where the uplift clears build cost comfortably, as in the Hampstead example, the investment case is stronger on its own terms. In both cases, the number that actually protects the value calculation is getting the waterproofing specification right the first time; a basement with a damp problem does not just fail to add value, it actively reduces it.
| Borough | Typical Property Value | Typical Conversion Cost | 10–20% Value Uplift |
|---|---|---|---|
| Fulham | £1.2m–£1.4m | £120k–£220k | £130k–£260k |
| Hampstead | £2.5m–£3.5m | £150k–£300k | £300k–£600k |
| Wandsworth | £1.1m–£1.3m | £100k–£200k | £120k–£240k |
| Wimbledon | £1.7m–£2.1m | £130k–£250k | £190k–£380k |
Figures above are indicative, based on typical property values and project scopes in each borough as of 2026, and are provided for planning purposes only. They are not a valuation of any specific property; a local surveyor or estate agent can confirm current values for your street.
Wondering what a basement conversion could be worth on your specific street?
Book a free consultation with a Sovran specialist, or use our cost calculator for an indicative build cost based on your property and scope.
Sovran is a RIBA-certified design and build company delivering basement conversions, home extensions and loft conversions across London since 2011. Our fixed-price model means the build cost quoted at the outset is the cost paid at completion.