Basement Conversion Cost London 2026

Basement conversion costs at a glance — London 2026

The table gives our published London ranges for each type of basement work. The rates per m² include structural works, tanking and waterproofing, mechanical and electrical installation and a standard fit-out. Anything outside that list, such as a lightwell or a party wall surveyor, is a separate line in your budget and is covered further down this page.

Cellar conversion cost London

If the house already has a cellar, converting it is the simplest and most cost-effective way to gain a room below ground, because the space exists and the job is to make it habitable. In London it typically costs £2,500–£4,000 per m², which puts a typical 30–40 m² cellar at £80,000–£150,000.

The money goes on four things: lowering the floor to achieve a minimum 2.4-metre ceiling height, by underpinning or benching; tanking the walls and floor; installing drainage; and the fit-out, which covers insulation, lighting, heating and ventilation. Depth matters. The further the floor has to drop, the more structural work sits beneath the finishes.

Planning is usually simpler too. A straightforward internal conversion that does not change the outside of the building generally falls within permitted development, so you can proceed without a planning application, provided there is no excavation outside the existing footprint, no lightwell and no change to the external fabric. The work is also less disruptive than a full excavation and can often be completed in 4–8 weeks on site. Cellars convert well into home offices, media rooms, guest bedrooms and utility spaces.

Underpinning to gain headroom

Where a cellar or partial basement has a low ceiling, deepening the existing foundations creates the headroom without a full excavation. Underpinning on its own typically costs £40,000–£80,000 and takes 8–12 weeks. It is a cost-effective way to turn a low-ceiling cellar into usable living space. Whether underpinning alone is enough depends on the depth of your foundations and the headroom you need, which we assess on a site visit.

Full basement excavation (dig-down) cost London

Where there is no cellar, or the one you have is too shallow, a full excavation creates an entirely new storey beneath the house. It costs £4,500–£6,500 per m², or £150,000–£300,000+ for a typical 30–40 m² basement.

The rate is higher than for a cellar conversion because of the additional structural engineering, underpinning, excavation and spoil removal. Most London houses sit on relatively shallow foundations, often only 600–900mm deep. To create a habitable basement they are taken down to at least 2.7–3.0 metres below ground level in a strict sequence, and a new reinforced concrete basement box is then built with full waterproofing.

The top figure is open-ended for a reason. A full-depth basement can hold open-plan living space, a home cinema, a gym, a wine cellar, a swimming pool or a self-contained flat, depending on planning consent and budget.

Two other things separate a dig-down from a cellar conversion when you budget. It will almost certainly need planning permission, and several boroughs, including Kensington and Chelsea, Camden and Westminster, restrict the depth, extent and duration of basement works, typically to a single storey beneath the house. It also takes 4–8 months on site, against weeks for a cellar.

Lightwells and other costs to budget for

A lightwell is an external excavation alongside the basement wall, usually at the front or rear, that brings daylight and ventilation into the rooms below. Each one adds £15,000–£25,000, depending on size. Lightwells almost always need planning permission because they change the outside of the property, and in a conservation area the design has to be handled carefully to secure consent.

The table lists the other costs with a published figure. Two of them come from our house extension guide and are shown for scale, because we do not publish a basement-specific figure for them.

Where planning permission is needed, add the application fee and the drawings that support it. A straightforward internal cellar conversion avoids both.

Party wall costs on a basement

Almost all basement conversions in London require Party Wall Act notices. The Act covers excavation within 3 metres of a neighbouring structure that goes deeper than its foundations, and within 6 metres where the excavation would cut a 45-degree line drawn from the bottom of those foundations.

If your neighbour consents in writing, no surveyor is needed. With a single agreed surveyor, expect £800–£1,500. Where each side appoints its own, £800–£3,500 per neighbour is typical, and a disputed matter that needs a third surveyor costs £3,000–£6,000+. As the building owner, you pay all reasonable surveyor costs, including your neighbour's surveyor. A statutory notice period of 1–2 months applies before work starts, and your neighbour has 14 days to respond to the notice. Our party wall agreement cost guide has the full breakdown.

What the cost per m² includes

Our published basement rates include structural works, tanking and waterproofing, mechanical and electrical installation and a standard fit-out. They exclude VAT and the other costs in the table above. On a full excavation, that covers these stages:

• Breaking out and excavation — the existing floor broken out, the ground dug by hand or mini-digger and the spoil removed by conveyor belt or skip
• Underpinning — the existing foundations extended to the new depth, typically in mass concrete, in a planned sequence of pins
• Basement structure — a new reinforced concrete floor slab and any new internal walls
• Waterproofing — typically a structurally waterproof concrete shell with an internal cavity drain membrane as a second line of defence
• Drainage — a sump chamber at the lowest point of the basement, with dual pumps, battery backup and high-level alarms as standard
• Mechanical and electrical first fix — wiring, pipework and ventilation ducting
• Insulation, plasterboard and plastering
• Second fix — sockets, switches, lighting and sanitary ware
• Flooring and decoration

What adds to the total: premium finishes, bespoke joinery, underfloor heating, MVHR ventilation systems and lightwells. Your itemised quote sets out exactly what is included for your project, so you can compare it line by line with any other quote you receive.

Waterproofing and drainage in the price

There are three ways to waterproof a basement. Type A is a waterproof membrane or coating on the external face of the walls and floor. Type B is a structure that is itself waterproof, typically a reinforced concrete box. Type C is an internal cavity drain membrane that collects any water and channels it to a sump pump, and it is the most common approach in London residential basements.

We typically specify Type B and Type C together, so the cavity drain membrane backs up a structurally waterproof shell. We also fit dual-pump systems with battery backup and high-level alarms as standard, because a pump failure in a basement is not something you want to find out about late.

What drives basement cost in London

The published ranges are wide because the same floor area can need very different amounts of work. These are the factors that move the figure.

• Type of work — converting an existing cellar costs £2,500–£4,000 per m²; excavating a new basement costs £4,500–£6,500 per m²
• Size — the published totals are for a typical 30–40 m² basement
• Depth and headroom — the deeper the dig below the existing foundations, the more underpinning and excavation the job carries
• Ground conditions and water — in many London locations a basement sits below the water table for at least part of the year, and London's ground conditions vary from site to site
• Waterproofing requirements — we typically specify a combined Type B and Type C system, a waterproof concrete structure with a cavity drain membrane inside it, rather than relying on one line of defence
• Spoil removal and access — excavated ground leaves by conveyor belt or skip, with significant lorry movements; as on any inner-London site, tight access adds time and cost to deliveries and waste removal
• Party walls — budget £800–£3,500 per neighbour for a party wall surveyor
• Planning and borough policy — full excavations and lightwells almost certainly need planning permission, and listed buildings and conservation areas need additional consents
• Fit-out specification — finishes, joinery, underfloor heating and MVHR ventilation all sit on top of a standard fit-out
• Whether you stay in the house — the programme for a full excavation depends partly on whether the property is occupied during the works

Basement cost against the value it adds

A full excavation costs more per m² than a house extension, which our extension cost guide puts at £2,500–£4,000 per m². So the useful comparison is with what the finished space is worth. London property values in desirable areas range from £8,000 to £15,000+ per m², while creating new basement space costs £2,500–£6,500 per m², usually well below the value it adds.

A well-executed basement conversion can add 10–20% to the value of your property, and in prime central London the uplift can be significantly more. Even at the top published rate of £6,500 per m², the cost is well under the price of property in an area where it sells for £10,000–£15,000 per m². For owners who need more room but do not want to leave the area, that is the sum that matters.

The return depends on quality. A poorly waterproofed basement, an inadequate structural design or a substandard fit-out does not add value; it creates problems. That is the reason to weigh quotes on what they include, not only on the bottom line.

How Alstruct prices a basement

We do not give a ballpark figure and then adjust it upwards once work is under way. We meet you on site and carry out a site investigation, including trial pits and soil analysis, before recommending an approach. We then review your drawings, or develop the design with our in-house architectural support, and take structural engineer input on the key elements.

You then receive a fixed-price, fully itemised quotation broken down by trade. The work is carried out under a JCT contract, which gives you a fixed price for the agreed scope, a defined programme and a clear procedure for variations. If the scope changes, because the ground reveals something unexpected or you decide to upgrade the specification, we confirm the additional cost in writing before any extra work is done.

If you already have drawings, send them with your enquiry. The contact form accepts PDF, JPG, PNG, HEIC and DWG files, up to 10 files of 25 MB each. Every project carries our 10-year workmanship warranty, which runs from practical completion and passes to the next owner if you sell.

How long a basement conversion takes

Programme length affects your budget, particularly if you plan to move out while the structural work is done. These are the published durations.

• Cellar conversion — typically 4–8 weeks on site
• Underpinning only — 8–12 weeks
• Basement with lightwells — 12–20 weeks
• Full excavation — 4–8 months on site, depending on size and complexity

On a full excavation, excavation and underpinning fall in months 1–3, the basement structure in months 2–4, and first fix, second fix and fit-out in months 4–6. Before any of that, allow 3–6 months for design, planning and structural engineering approvals. From first contact to completion, a full basement project typically takes 8–14 months.

Party wall notices should go out as early as possible, ideally at the same time as the planning application, so the process does not delay the start on site. For a cellar conversion, most homeowners stay in the house throughout. For a full excavation, some choose to move out temporarily, because underpinning and excavation are disruptive.

The figures on this page are indicative London ranges for 2026 and exclude VAT. Actual costs depend on ground conditions, depth, waterproofing requirements and specification, and your own price is fixed in a written quote after a site visit.

Frequently Asked Questions

Prices shown are indicative ranges based on London market data for 2026 and exclude VAT. Actual costs depend on the depth, access, ground conditions, structure and specification of your property. Every Alstruct quote is a fixed, itemised price prepared after a site visit.