Can a neighbour block my basement?

No. A neighbour cannot veto your basement. If a neighbour dissents under the Party Wall etc. Act 1996, that triggers a party wall award, the document that sets out the agreed works and the neighbour's protections. The award controls how you build near their wall, but it does not stop the basement. Planning is decided by the council under Policy CD11, not by your neighbours.

How long does a basement take to build?

It depends on the job type. On site, a cellar conversion usually runs about six to fourteen weeks. A new single-storey dig, with underpinning, excavation and waterproofing, more commonly takes several months to around a year, since the structural work must be done in strict, cured sequence. A larger or multi-level basement runs longer again. Design, planning and the two-month party wall notice come first and add months before building starts. These are indicative build ranges, not a programme for your project.

What happens if the waterproofing fails?

A drained system floods if its pump fails. The most common habitable-basement method is a Type C, or cavity drain, system, which does not block water but collects it behind a membrane and pumps it away. That makes the pump critical. The correct specification is a duty-and-standby pump, meaning a second pump that takes over automatically, with battery backup for a power cut. Without that, a single pump failure or power cut can flood the room.

Do I need planning permission for a basement in Kensington and Chelsea?

Yes. Essentially every basement in Kensington and Chelsea needs full planning permission. A 2016 Article 4 direction, which is a council order that removes the automatic right to carry out certain works, took away the permitted-development route for basements across the borough. So you cannot rely on permitted development, and your basement is judged against Policy CD11, the council's basements policy, whatever its size.

Why is a basement more expensive per square foot than an extension or a loft?

Because you pay to hold the ground and the neighbour's house up before you build anything. An extension or a loft conversion works with the structure that is already there. A basement has to create new, stable ground first: excavation, underpinning, temporary works and waterproofing all come before the first usable square foot exists. An extension or loft carries none of that fixed, below-ground cost, so its rate per foot is far lower.

Basement Cost Per Square Foot in London 2026

What a basement costs per square foot in prime London

Basement cost per square foot in prime central London runs, as an indicative market range, from about £110 per square foot for a simple cellar conversion to about £700 per square foot or more for a fully finished new dig. The spread is that wide because a basement covers very different jobs, and the honest answer splits by job type before it means anything. These are market ranges gathered from contractor and architect guidance, not a firm cost dataset, and not a quote.

Job type	Per square metre (indicative)	Per square foot (indicative)
Converting an existing cellar	£1,200 to £2,500	£110 to £232
New single-storey dig, structural shell	£3,000 to £5,000	£280 to £465
New single-storey dig, finished	£4,500 to £7,500+	£420 to £700+
Dig under the garden	£1,500 to £2,800	£140 to £260

Two terms explain the biggest gap in that table. The structural shell, sometimes called the box, is the excavation, underpinning, concrete structure and waterproofing: the hole made safe and dry. The finished figure adds the fit-out, meaning everything that turns the box into a room, from floors and services to lighting and joinery. A prime specification, with a pool or a deep dig, can push the finished figure towards £900 per square foot in Chelsea commentary, though that is the high end, not the norm.

All the figures above exclude VAT, professional fees and fit-out, which are covered later and move the total materially. A multi-storey or iceberg basement is a different world again, costing from around £500,000 into the millions, but it sits outside this article: in Kensington and Chelsea the current planning rules rarely permit one.

Why per square foot is the wrong question for a basement

Per square foot is the wrong tool for a basement because its largest costs are fixed, not proportional to floor area. A kitchen or a loft scales with size: double the area and you roughly double the materials and labour. A basement does not.

The reason is the spine of the whole subject. A basement is not a floor. It is a hole held open against clay, water and the house next door. The three things that set its price are the ground, the party wall and the council, and none of them scale with floor area. You underpin the house whether the new room is 20 square metres or 40. You serve the same party-wall notices on the same neighbour. You satisfy the same council policy. The digging and finishing scale with size, but the expensive, fixed work does not.

So the per-square-foot figure takes a cost that is mostly fixed and spreads it across a floor. That flatters a large basement, where the fixed cost is shared over more space, and punishes a small one, where the same fixed cost is carried by a few square metres. Two basements quoted at the same rate per foot can hide completely different jobs. It is the figure that tells you least, even though it is the one people ask for first.

What actually sets the price: the ground, the party wall, the council

Three forces set a basement's price, and floor area is not one of them: the ground, the party wall and the council. Each adds cost that a per-foot figure cannot capture.

The ground is the largest. On a tight terraced street, excavation means hand-digging and grab lorries rather than a free machine dig, and the skips, muck-away and permits alone can reach £3,000 to £10,000. London homes often have foundations only 600 to 900 millimetres deep, so underpinning, extending those foundations deeper so the ground beneath can be removed safely, is almost always needed, held by temporary works, the propping and shoring that keep the hole open. London clay also holds a perched water table, a layer of groundwater trapped above the clay, so the box must be waterproofed as a system, not a coating. Cementitious tanking, which seals the structure, runs roughly £60 to £100 per square metre, and a cavity drain membrane, which collects water and channels it to a pump, roughly £120 to £220 per square metre. Excavation and underpinning together are commonly 30 to 40 per cent of the whole construction cost.

The party wall is the second. Under the Party Wall etc. Act 1996, if you excavate within three metres of a neighbour's structure and deeper than their foundations, you must serve notice at least a month before starting. Each adjoining owner can appoint a surveyor, and you pay all the reasonable fees, typically £1,500 to £3,000 per neighbour, and more where each side appoints its own.

The council is the third. In Kensington and Chelsea, a basement is judged against Policy CD11 of the Local Plan, which caps it at one storey and 50 per cent of the garden, forbids excavation under a listed building, and requires drainage, traffic and structural method documents with the application. It also needs a Basement Impact Assessment, the engineering report on how the dig affects the building and its neighbours, explained in the Basement Impact Assessment guide. Every one of these is a cost a small basement carries in full.

The two lines optimistic quotes leave out: VAT and contingency

Two lines decide whether a per-square-foot figure is honest, and both are routinely left off: VAT and contingency. Together they move a headline rate by roughly a third.

Start with VAT. Most domestic basement work, on an occupied home, is standard-rated at 20 per cent. Professional fees, the architect, engineer and surveyor, are always charged at 20 per cent. The 5 per cent reduced rate exists, but it applies only in narrow, evidenced cases: mainly a home not lived in during the two years before work starts, or work that changes the number of dwellings. For a standard occupied prime-London basement, assume 20 per cent on both the build and the fees. Any quote that models 5 per cent without a specific qualifying reason is not being straight with you.

Then contingency, the sum set aside for what the drawings cannot predict. For below-ground work it should be 15 to 20 per cent, higher than for any other domestic project. The reason follows the spine: clay, water and old foundations spring surprises found only once the ground is opened, and those surprises do not scale with floor area. A small basement is not a small risk.

Put the two together and the arithmetic is unforgiving. A shell quoted at, say, £4,000 per square metre is really closer to £4,800 with VAT, before a penny of contingency or a single fee. That is why a proper cost plan matters more than a rate per foot: it prices the fixed and variable parts separately, with VAT and contingency in plain sight rather than hidden in one number.

What changes the number by postcode

Location changes a basement's cost through real, physical drivers, not a per-postcode price multiplier, because no such published table exists. What varies by address is the ground, the neighbours, the street and the heritage status, each with a real cost attached.

Ground conditions vary street by street. London clay over gravel is the norm, but the depth of clay, the height of the water table and old watercourses all change the underpinning and waterproofing spec. A site near a historic river needs more robust pumping than one on higher, drier ground.

Party-wall density varies by building type. Most of prime central London is terraced or semi-detached, so almost every basement triggers the Party Wall etc. Act 1996, and every adjoining owner adds legal and surveyor cost. A rare detached house triggers far less of it. The number of neighbours you dig next to is a location factor with a direct price.

Access varies by street: a wide street with room for machinery is cheaper to work than a narrow terrace that forces hand-digging and grab access. Heritage status varies too. Much of Kensington and Chelsea is covered by conservation areas, and a basement under a listed building is not permitted at all, so across the borough the design is often narrowed before anyone digs. None of this is a postcode multiplier. It is a set of physical facts about your address that a real cost plan prices one by one.

The number people ask for, and the number they need

The honest answer to what a basement costs per square foot is always a range and a method, never a single rate. The trade gives one anyway, because clients ask for one, and because a single number is far easier to sell than a range and a method. That is the quiet incentive behind every tidy per-foot figure.

Any builder in prime London, Myrmex included, is asked for one number, and the temptation is always to give it. The honest job is to refuse the false comfort: to say that the price is set by the ground, the party wall and the council, that it splits by job type, and that VAT and contingency sit on top. A number without that method behind it is a guess dressed as a quote.

None of this makes a basement unknowable. It makes it pricable properly, by job type and by the drivers that move the figure, not by a rate that flatters a big dig and punishes a small one. The useful question is not what a basement costs per square foot. It is what my basement costs, on my street, at my scale, once the ground, the neighbours, the council, VAT and contingency are counted. If you want to work through what your own basement is likely to cost, that is the next step.

This is general information and an illustrative set of market ranges, not a quote or a cost plan. Every basement, site and specification is different, and the real number comes from a cost plan built around your specific project, not from a rate per square foot.

Basement Cost Per Square Foot in London: What the Number Hides