What a basement conversion involves
Most London houses sit on shallow foundations and a basement conversion either uses the existing cellar (if there is one) or creates new space by underpinning the existing walls and digging down. Underpinning is a structural-engineering operation done in numbered sequence under continuous monitoring. Once the new structural shell is complete, the basement is tanked (waterproofed) using either a Type A external membrane, Type B integral concrete or Type C cavity drainage system, depending on water table, ground conditions and end use. Habitable basements need natural light (lightwells), mechanical ventilation, fire escape, electrics to current zone-of-influence regulations and proper drainage with pumped sumps.
Typical basement conversion scope
Existing-cellar conversions (where the basement exists but is uninhabitable) typically run from £45,000 to £90,000 and take 8 to 14 weeks. Full dig-out basements (creating new space below ground) typically run from £180,000 to £420,000 and take 16 to 30 weeks. Double-storey basements with lightwells in Notting Hill or Holland Park can exceed £600,000 and run 9 months on site. The price difference is structural — underpinning, propping and excavation account for over half the cost on a full dig-out.
Why the structural detail matters
Bad basement conversions fail in two ways: water ingress (poor tanking) and structural settlement (poor underpinning). Both are expensive to remediate and most of the time impossible to fix without re-doing the basement. We use only chartered structural engineers for design, only experienced contractors for underpinning, and we install Type B and Type C waterproofing in series rather than relying on a single barrier. Our basements come with a 10-year structural warranty backed by Premier Guarantee.
Building control and party wall
Basement conversions in London are subject to enhanced building control oversight. Many boroughs (Camden, Westminster, Kensington and Chelsea) have specific basement policies that limit excavation depth and number of underground storeys. Party wall agreements are required with every adjoining owner and can take 6 to 14 weeks to complete before excavation can start. We coordinate all of this as part of our project management.
Basement conversion cost in London 2026
Basement conversion is the most expensive square metre of any residential extension type because most of the cost goes into structural work below ground rather than visible finishes. The figures below are taken from KRAMAN basement projects completed in the last five years across Leytonstone, Brockley, Camden and Notting Hill. All exclude VAT.
| Basement type | Typical London price | Per square metre |
|---|---|---|
| Existing cellar conversion (no dig) | £25,000 to £45,000 | £1,800 to £3,000 |
| Semi-basement (partial dig, partial lower ground) | £40,000 to £90,000 | £3,000 to £4,500 |
| Full basement (new dig, full headroom) | £80,000 to £150,000 | £4,000 to £7,000 |
| Lightwell addition | £15,000 to £25,000 | included in build |
| Party wall agreements (typical 2 neighbours) | £5,000 to £10,000 | project disbursement |
Cost is driven by depth of dig (each extra metre adds 18 to 25 percent), structural condition of the existing house, water table, ground type (London clay is forgiving; alluvial gravels along the Thames are not), and presence of trees within zones of influence. Underpinning typically accounts for 35 to 50 percent of total cost on a full dig-out; waterproofing and drainage another 15 to 22 percent.
Premier Guarantee 10-year structural warranty is included in our basement quotes. Listed and conservation-area basement conversions sit at the top of the band because of specification controls and longer planning timescales.
How long does a basement conversion take?
Basement programmes vary enormously by scope. An existing cellar conversion runs 8 to 14 weeks; a full new dig-out runs 16 to 30 weeks. The phased breakdown for a typical 30 square metre full basement under a London Victorian terrace looks like this.
| Phase | Duration | What happens |
|---|---|---|
| 1. Drawings, planning, party wall | 12 to 22 weeks (pre-site) | Structural design, planning application, party wall agreements with each neighbour |
| 2. Site set-up, protection | 1 to 2 weeks | Scaffolding, propping, dust line, neighbour communication |
| 3. Underpinning sequence | 6 to 10 weeks | Numbered pin sequence, concrete cures between pours; monitored continuously |
| 4. Excavation, slab, retaining | 3 to 5 weeks | Dig down to required level, lay reinforced slab, retaining walls |
| 5. Tanking and drainage | 2 to 4 weeks | Type C cavity drain membrane, perimeter channels, sump and pump installation |
| 6. M&E first fix | 1 to 2 weeks | Electrical, plumbing, MVHR ventilation, underfloor heating manifolds |
| 7. Internal finishes | 2 to 4 weeks | Insulation, plasterboard, plaster, joinery, decoration |
| 8. Building control sign-off | 1 to 2 weeks | Final inspection, structural sign-off, BS 8102 waterproofing certificate |
Total typical timeline: 16 to 24 weeks on site, plus 12 to 22 weeks of pre-construction. Double basements (two underground storeys) extend the on-site phase to 9 to 12 months. Habitability above the basement is restored once the underpinning is complete and temporary propping is replaced with the permanent retaining walls — usually 8 to 10 weeks into the site programme.
Basement vs cellar conversion: which one do you have?
The terms basement and cellar are used interchangeably in conversation but the building work is completely different. A cellar conversion uses existing below-ground space; a basement conversion creates new below-ground space by digging out. The cost difference is roughly 4 to 6 times.
| Aspect | Cellar conversion | Basement conversion |
|---|---|---|
| Starting condition | Existing cellar / coal store, typically 1.6 to 2.0 metres headroom | No useful below-ground space, or unstable existing cellar to be replaced |
| Structural work | Minimal; surface preparation, lightwell if natural light needed | Major; full underpinning of existing house, retaining walls, slab |
| Headroom achievable | Limited to existing (often 1.8 to 2.1 metres after damp work) | Designed to 2.4 to 2.7 metres clear |
| Typical use | Utility, gym, wine store, occasional guest room | Habitable bedroom, family room, cinema, full additional storey |
| Cost | £25k to £45k | £80k to £150k+ |
| Programme | 8 to 14 weeks | 16 to 24 weeks |
A cellar conversion is the cheaper option if you already have the space and accept reduced ceiling height. A full basement conversion is the right answer if you need a habitable bedroom or family room. On a free site visit we confirm which type your property supports and quote both options where viable.
Waterproofing and Building Regulations
A basement that lets water in is a basement that has failed. London water tables vary by borough — clay-based ground in Hackney and Leytonstone is benign; alluvial Thames gravels in Greenwich, Wapping and Battersea can be saturated. Waterproofing is designed under British Standard BS 8102:2022 to suit the worst-case condition.
Three waterproofing system types
- Type A (external membrane): bituminous or HDPE membrane applied to the outside of the structure. Reliable on new-build but impractical to retrofit because it requires the entire perimeter to be re-excavated.
- Type B (integral concrete): the structural concrete itself is the waterproof barrier, designed with water-stopping admixtures and crack-controlled detailing. Common on new-build basements.
- Type C (cavity drain membrane): studded membrane fixed to the internal face of the walls and floor, with channels at the perimeter, draining to a sump and pump. Standard for converting existing or underpinned basements; the only practical choice for retrofit work.
Belt and braces: Type B and Type C in series
On every full basement conversion we install Type B integral waterproof concrete AND Type C cavity drain in series rather than relying on a single barrier. The cost premium over single-system waterproofing is roughly 8 to 12 percent of the basement budget, and the failure mode in either system is benign — Type C drains water to the sump, Type B holds it back. Either one alone could leak; the combination almost never does.
Sump pumps and drainage
Twin sump pumps (one primary, one backup) installed in a perimeter channel system. Battery-backed alarm on rising water level. Mains-powered pumps with battery override during power cuts. The pump system is one of the most common warranty items on a basement conversion; we service the pumps annually as part of our handover programme.
Structural underpinning
Underpinning means temporarily transferring the load of the house onto a new lower foundation pin by pin. Each pin is typically 1 metre wide and is dug, cast and concreted in sequence so that no more than three adjoining pins are open at any time. The sequence is designed by a chartered structural engineer and inspected by building control at each stage. On a typical Victorian terrace we have 18 to 28 pins. The work is slow and uncomfortable but produces a stable platform for the new basement floor.
Frequently asked questions
Can I dig down under a Victorian terrace?
Usually yes, but the structural design is more involved than a 1930s semi. We work with structural engineers who specialise in heritage-property underpinning to keep the original brickwork stable.
How long before the upper floors are usable again?
During underpinning the rooms directly above the works are unusable due to noise and vibration. A typical full dig-out makes the ground floor usable within 8 to 10 weeks of starting on site once temporary propping is in place.
Do I need planning permission?
Many councils now require planning consent for any new basement, even where the conversion is invisible from outside. Central London boroughs almost always require planning. We handle the application as part of the project management.
How much does a basement conversion cost in London?
Existing cellar conversion: £25,000 to £45,000. Semi-basement: £40,000 to £90,000. Full basement (new dig-out): £80,000 to £150,000. Double basement or central London listed property: £200,000 to £600,000+. Add £15,000 to £25,000 for a lightwell and £5,000 to £10,000 for party wall agreements.
How long does a London basement conversion take?
A cellar conversion runs 8 to 14 weeks on site. A semi-basement runs 12 to 18 weeks. A full new-dig basement runs 16 to 24 weeks on site, plus 12 to 22 weeks of pre-construction for planning, structural design and party wall agreements. Double basements run 9 to 12 months on site.
What is the difference between a basement and a cellar conversion?
A cellar conversion uses existing below-ground space (typically a Victorian coal cellar). Cost: £25k to £45k. A basement conversion creates new below-ground space by underpinning the existing house and digging down. Cost: £80k to £150k+. The structural complexity is much higher on a basement conversion.
What waterproofing system do you use?
We install Type B integral waterproof concrete plus Type C cavity drainage membrane in series on every full basement conversion. Twin sump pumps, battery backup, alarm on rising water level. Designed to BS 8102:2022 and covered by 10-year Premier Guarantee structural warranty.
Do I need planning permission for a basement?
Most London boroughs now require planning permission for any new basement, even where the conversion is invisible from outside. Central London boroughs (Camden, Westminster, Kensington and Chelsea) have specific basement policies that limit depth, light wells and number of storeys. We handle the application as part of project management.
Related projects
Ready to discuss your project?
Free initial site visit. Written rough-order-of-magnitude budget within three working days.
07717 778 612 Or send a brief →