Basement Conversions London
Create premium below-ground living space with expert basement excavation, underpinning and waterproofing across London.
Basement conversion and excavation contractors in London
Alstruct converts existing cellars, excavates new basements beneath London houses, forms lightwells and underpins foundations. We manage every stage: site investigation and trial pits, architectural and structural design, planning and party wall matters, excavation, underpinning, waterproofing, mechanical and electrical installation and the final fit-out.
A basement is a structural engineering project before it is anything else. Every element is designed by a qualified structural engineer, and the ground conditions, the water table and the neighbouring foundations decide what is feasible. We investigate the site, with trial pits and soil analysis, before we recommend an approach.
You receive a fixed-price, itemised written quote after a site visit, a JCT contract as standard and a 10-year workmanship warranty. We are CHAS Elite and Constructionline Gold accredited, members of the Federation of Master Builders, and carry £10M public liability insurance.
At a glance: a cellar conversion in London typically costs £2,500–£4,000 per m², or £80,000–£150,000 for a typical 30–40 m² space. A full dig-down excavation costs £4,500–£6,500 per m², or £150,000–£300,000+. Underpinning on its own typically costs £40,000–£80,000 and each lightwell adds £15,000–£25,000, all excluding VAT. A straightforward cellar conversion can often be completed in 4–8 weeks on site; a full excavation takes 4–8 months. Converting an existing cellar without external changes generally falls within permitted development, while full excavations and lightwells will almost certainly need planning permission. Every published figure is set out in our basement conversion cost guide.
Types of basement work we do
The right route depends on whether the house already has a cellar, how deep the foundations are and how much headroom you need.
Conversions and excavations
Cellar conversion
We turn an existing cellar or vault into a habitable room: lowering the floor to achieve a minimum 2.4-metre ceiling height, tanking the walls and floor, installing drainage, then fitting insulation, lighting, heating and ventilation. It suits the Victorian and Edwardian terraces where cellars are common. Typical cost is £2,500–£4,000 per m², or £80,000–£150,000 for 30–40 m². A straightforward conversion can often be completed in 4–8 weeks on site.
Full excavation (dig-down)
Where there is no cellar, or the existing one is too shallow, we excavate beneath the existing foundations, underpin the whole structure in a planned sequence and build a new reinforced concrete basement box with full waterproofing. It creates the most space and the most freedom in layout. Typical cost is £4,500–£6,500 per m², or £150,000–£300,000+ for 30–40 m², with 4–8 months on site.
Lightwells and underpinning
Lightwells
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. It can include a staircase giving direct access to the garden, and is often built with a cellar conversion or a full excavation. Each lightwell adds £15,000–£25,000 and almost always needs planning permission. Allow 12–20 weeks for a basement with lightwells.
Underpinning
We deepen the existing foundations to gain headroom in a low cellar or partial basement, without a full excavation. The work is typically done in mass concrete, cast as a sequenced series of pins so the structure stays stable throughout. Underpinning on its own typically costs £40,000–£80,000 and takes 8–12 weeks.
What a basement conversion costs in London
These are our published London ranges for 2026. They include structural works, tanking and waterproofing, mechanical and electrical installation and a standard fit-out.
| Type of work | Cost per m² | Typical total | Time on site |
|---|---|---|---|
| Cellar conversion, 30–40 m² | £2,500–£4,000 | £80,000–£150,000 | 4–8 weeks |
| Full excavation (dig-down), 30–40 m² | £4,500–£6,500 | £150,000–£300,000+ | 4–8 months |
| Underpinning only | n/a | £40,000–£80,000 | 8–12 weeks |
| Lightwell | n/a | £15,000–£25,000 each | 12–20 weeks with lightwells |
Indicative London figures for 2026, excluding VAT. Your price depends on ground conditions, depth, waterproofing requirements and specification, and is fixed in a written quote after a site visit.
A full excavation costs more per m² than a cellar conversion because of the additional structural engineering, underpinning, excavation and spoil removal. Beyond the type of work, the figure moves with the size of the basement, the depth you need, the ground conditions and the waterproofing the site calls for. Spoil leaves by conveyor belt or skip, so tight inner-London access adds time and cost, as it does on any build.
Specification is the other lever. Higher-specification finishes, made-to-measure joinery, underfloor heating, MVHR ventilation and lightwells all add to the total. Party walls carry their own cost: almost every London basement needs Party Wall Act notices, and a surveyor typically costs £800–£3,500 per neighbour, paid by you as the building owner. Our basement conversion cost guide explains what the rate per m² includes, what drives it and how we price a job.
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We visit, measure and send an itemised quote on a JCT contract. If you have drawings or photos, send them with your enquiry: the form takes PDF, JPG, PNG, HEIC and DWG files, up to 10 files of 25 MB each.
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How a basement project runs
The sequence below is for a full excavation. A cellar conversion is a shorter version of the same steps.
- Site visit and ground investigation. We survey the property, assess feasibility, open trial pits and analyse the soil. Ground conditions, the water table, the depth of your foundations and the distance to your neighbours' foundations decide which approach is realistic.
- Structural and architectural design. A qualified structural engineer designs every structural element, and the waterproofing strategy is set at the same stage. With in-house architectural support, we prepare the drawings for planning, where it is needed, and for building regulations approval.
- Planning and party wall awards. Party wall notices go out as early as possible, ideally alongside the planning application, so the awards do not hold up the start. We carry out condition surveys on the adjoining properties and put a Construction Management Plan in place.
- Breaking out and excavation. The existing floor is broken out and the ground is dug by hand or mini-digger, with spoil taken out by conveyor belt or skip. On a full excavation this falls in months 1–3 and is the most disruptive part of the job, with significant noise and lorry movements.
- Underpinning in sequence. London houses often sit on foundations only 600–900mm deep. For a habitable basement they are taken down to at least 2.7–3.0 metres below ground level, typically in mass concrete pins cast alternately on opposite sides of the building, so it stays stable at every stage.
- Slab and structure. With excavation and underpinning complete, the new reinforced concrete floor slab is poured and any new internal walls are built. On a full excavation this runs through months 2–4.
- Waterproofing and drainage. We typically specify a combined system: a structurally waterproof concrete shell (Type B) with an internal cavity drain membrane (Type C) as a second line of defence. Water is channelled to a sump at the lowest point, and we fit dual pumps with battery backup and high-level alarms as standard.
- Fit-out. First-fix wiring, pipework and ventilation ducting go in, followed by insulation, plasterboard and plaster, second fix, flooring, decoration and joinery. On a full excavation this is months 4–6.
- Building control sign-off and handover. The work is signed off by building control. Our 10-year workmanship warranty runs from practical completion and passes to the next owner if you sell.
Planning, party walls and building regulations
Planning permission
A straightforward internal cellar conversion that leaves the outside of the building unchanged generally falls within permitted development. 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.
Planning permission is required for a full excavation that extends beyond the footprint of the house, for front or rear lightwells that change the external appearance, for any work to a listed building (which also needs listed building consent), in some conservation areas, and where the basement will be used as a separate dwelling. Several boroughs, including Kensington and Chelsea, Camden and Westminster, have their own basement policies. These restrict depth, extent and duration, and typically limit a basement to a single storey beneath the house.
Party walls and neighbours
Almost all London basement conversions need 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. We prepare and serve the notices and coordinate with the appointed surveyors. Fees and timings are in our party wall agreement cost guide.
Building regulations
Building regulations approval is in place before work starts on site, alongside the structural design and the waterproofing strategy, and the completed basement is signed off by building control. For the full process, read our basement conversion guide.
What is included in every basement conversion
- Free site visit and feasibility assessment
- Ground investigation and trial pits
- Architectural and structural engineering design
- Planning permission applications
- Party wall notices and surveyor coordination
- Excavation and spoil removal
- Underpinning and structural reinforcement
- Waterproofing and tanking systems
- Mechanical ventilation and dehumidification
- Electrics, plumbing and heating
- Plastering, flooring and decoration
- Full project management throughout
- Fixed-price, itemised quote on a JCT contract
- 10-year workmanship warranty
Related projects: underpinning and below-ground work
Our published case studies are renovations and extensions rather than basement conversions. Two of them include the structural work a basement depends on: underpinning, a lowered floor, new foundations and below-ground tanking.
Areas we cover for basement work
We work across London. Basement conversions are a routine part of residential construction in Hampstead, Kensington, Chelsea and Islington, and lower-ground and basement projects are among the usual ways to extend in Notting Hill. In Camden, lower-ground dig-outs sit alongside rear extensions as the typical project. In Fulham and Hammersmith, a lower-ground dig-out is often combined with a side-return kitchen extension. Clapham sees basement dig-outs near the Common, and Hackney basement conversions on its taller Georgian and Victorian houses. See all the areas we cover.
Dealing with damp in an existing cellar or lower ground floor? Our guide to basement and cellar damp: tanking, injected DPC or membrane explains how it is diagnosed and fixed.
Frequently Asked Questions
How much does a basement conversion cost in London in 2026?
A cellar conversion in London typically costs £2,500–£4,000 per m², or £80,000–£150,000 for a typical 30–40 m² space. A full excavation (dig-down) costs £4,500–£6,500 per m², or £150,000–£300,000+. Underpinning on its own typically costs £40,000–£80,000 and each lightwell adds £15,000–£25,000. These figures exclude VAT. The final figure depends on ground conditions, depth, waterproofing requirements and specification, and we fix it in a written, itemised quote after a site visit.
Do I need planning permission for a basement conversion in London?
A straightforward internal conversion of an existing cellar that does not alter the outside of the property generally falls within permitted development and does not need planning permission. Full basement excavations, lightwells and any work that changes the external appearance will almost certainly need it, and listed buildings and conservation areas need additional consents. Several boroughs, including Kensington and Chelsea, Camden and Westminster, also have their own basement development policies.
How long does a basement conversion take?
A straightforward cellar conversion can often be completed in 4–8 weeks on site. Underpinning on its own takes 8–12 weeks, and a basement with lightwells 12–20 weeks. A full excavation takes 4–8 months on site, depending on size and complexity. Including design, planning, structural engineering and party wall agreements beforehand, a full basement project typically takes 8–14 months from first contact to completion. You receive a detailed programme before work begins.
Is my house suitable for a basement conversion?
Most London properties can take a basement conversion, but suitability depends on ground conditions, the level of the water table, how close the neighbouring foundations are and local planning policy. Houses with an existing cellar, common in Victorian and Edwardian terraces, are the simplest to convert. We carry out a site investigation, including trial pits and soil analysis, before recommending an approach.
Do I need a party wall agreement for a basement conversion?
Almost certainly. 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 dig would cut a 45-degree line drawn from the bottom of those foundations. Notices should go out as early as possible, ideally with the planning application. A party wall surveyor typically costs £800–£3,500 per neighbour, paid by the building owner. We prepare and serve the notices for you.
Will a basement conversion disturb my neighbours?
Basement excavation involves significant groundworks, so there will be noise, vibration and lorry movements while spoil is removed, mostly during the excavation and underpinning phase. We serve party wall notices where required, carry out condition surveys on the adjoining properties and run the site under a Construction Management Plan to keep disruption to a minimum.
Can I live in the house during a basement conversion?
For a cellar conversion, most homeowners stay in the property throughout, although there will be noise, dust and some disruption. For a full excavation it depends on the extent of the work: underpinning and excavation are disruptive, and some homeowners choose to move out temporarily. We plan the works to minimise disruption and advise on the best approach for your project.
How do you keep a basement dry?
There are three approaches to basement waterproofing: a waterproof membrane or coating on the external face of the walls and floor (Type A), a structure that is itself waterproof, typically a reinforced concrete box (Type B), and an internal cavity drain membrane that channels any water to a sump pump (Type C). We typically specify a combined Type B and Type C system, and we install dual-pump systems with battery backup and high-level alarms as standard.