Basement conversions in North London
Converting cellars, lowering floors and excavating new basements: structure, waterproofing and light designed together.
A basement conversion creates habitable space below a house — by converting an existing cellar, by lowering a cellar floor to gain head height, or by excavating a new basement under the house or garden. It is the most technically demanding domestic project, involving underpinning, structural steel, a designed waterproofing system, drainage pumps and careful management of neighbours' foundations under the Party Wall Act.
Done well, it adds a whole floor — a family room, cinema, gym, guest suite or self-contained annexe — on a plot where extending outwards or upwards is not possible. Milkov & Son plans and builds basements in North London with the structural engineer and waterproofing designer involved from the first drawing.
| Route | What it involves | Head height gained | Typical cost / m² (2026) |
|---|---|---|---|
| Cellar conversion | Waterproofing, insulation, floor, ventilation and services in an existing cellar with adequate height | Existing (usually 2.0–2.3 m) | £2,800 – £4,200 |
| Floor lowering | Underpinning the walls in sequence and digging down to give 2.4 m+ clear height | Typically +0.5–1.0 m | £4,000 – £5,500 |
| New basement | Full excavation under the house and/or garden with a new reinforced-concrete box, light wells and a new stair | Full storey, 2.4–2.7 m | £4,800 – £6,500+, higher on difficult sites |
Ranges from LCCL, Architecture for London and BookaBuilderUK 2026 basement cost guides; excluding VAT, fees and fit-out. See the cost section below.
Lowering or excavating means the house's foundations must be taken deeper. This is done by underpinning — excavating and casting new concrete beneath the existing footings in a "hit and miss" sequence of short bays so the wall is never unsupported — or, on larger schemes, by piling and a reinforced-concrete box. Steel beams and columns carry the ground floor while the space beneath is opened up; temporary props and monitoring protect the house and the neighbours' walls. The structural engineer designs and inspects each stage, and building control inspects the underpinning bays before they are backfilled. Our site photographs show this sequence: props and steels in, cavity membrane on.
Basements below the water table or in North London clay will see water. BS 8102:2022, the code of practice for protecting below-ground structures, describes three types of protection — Type A (barrier or "tanking"), Type B (structurally integral, i.e. the concrete itself), and Type C (drained cavity: a studded membrane on the walls and floor that channels any water to a sump and pump). For conversions we normally combine two types, most often a Type C cavity-drain system with a sump pump and battery back-up, installed by a specialist whose work is guaranteed and insured. Waterproofing is designed by a qualified waterproofing designer, not left to the last trade in.
Converting an existing cellar without changing the outside of the house is usually not development at all. Excavating a new basement, lowering floors substantially, or adding light wells and external stairs is normally an engineering operation that needs a planning application, and several North London boroughs (Camden, Haringey, Westminster and Kensington & Chelsea most strictly) have basement-specific policies limiting depth, extent under gardens and construction-management requirements. Barnet assesses basements case by case with attention to trees, drainage and neighbours' amenity. A Basement Impact Assessment or structural method statement is often required with the application. In conservation areas, light wells and front railings are scrutinised.
Basements are the strongest case for the Party Wall etc. Act 1996: excavating within 3 m of a neighbour's foundations and below their base (or within 6 m where a 45° line from their foundations is crossed) requires a 3/6-metre notice one month before starting, with drawings, and neighbours can require their foundations to be underpinned or strengthened at your cost. A schedule of condition of the neighbours' property before work, and movement monitoring during it, protect everyone. We build these steps and their timescales into the programme.
The difference between a basement that is used every day and one that is not is daylight. Light wells at the front, a glazed link or sunken courtyard at the rear, a stair with a rooflight above it, and — on sloping sites — a lower-ground floor that walks out to the garden. Building Regulations require ventilation, and a habitable basement needs a protected escape route or an escape window/door to a light well large enough to climb out of. Read how to maximise natural light in a basement.
Cost guide · figures current as of September 2026
| Item | Typical range | Notes |
|---|---|---|
| Cellar conversion (existing height), per m² | £2,800 – £4,200 / m² | |
| Floor lowering with underpinning, per m² | £4,000 – £5,500 / m² | |
| New basement excavation, per m² | £4,800 – £6,500+ / m² | Difficult sites, deep water tables and premium specification can reach £8,000–£10,000 / m² |
| Typical 40 m² new basement under a terrace | £220,000 – £320,000 | |
| Waterproofing system with sump and pumps | £12,000 – £30,000 | Included in per-m² ranges; shown for scale |
| Fit-out (bathroom, kitchenette, joinery, finishes) | £20,000 – £80,000 | |
| Design, engineering, waterproofing design, planning, party wall | 10 – 15% of build cost |
North London market ranges excluding VAT, fees and fit-out unless stated. Basements have the widest range of any project because ground conditions, water and access dominate the cost; we do not give a figure without a site visit and, usually, a trial pit or borehole.
What could your basement conversions project cost?
Use our project calculatorSources: Architecture for London — Basement construction costs in London (2026); BookaBuilderUK — Basement conversion cost 2026; GOV.UK — Party walls and building work. Market guides only — not a Milkov & Son quotation.
Stages overlap where they can. We agree a programme in writing before work starts — see how a project runs.
In 2026, converting an existing cellar costs around £2,800 to £4,200 per square metre, lowering a floor £4,000 to £5,500, and excavating a new basement £4,800 to £6,500 or more, all excluding VAT, fees and fit-out. A 40 m² new basement under a terrace is typically £220,000 to £320,000.
Converting an existing cellar without external changes usually does not. New basements, substantial floor lowering, light wells and external stairs generally need a planning application, and several North London boroughs have specific basement policies and require a Basement Impact Assessment.
By a waterproofing system designed to BS 8102, usually a Type C cavity-drain membrane with a sump and pumps, often combined with a second form of protection. It is designed by a waterproofing specialist and installed with an insurance-backed guarantee.
Not if it is engineered, sequenced and monitored properly. The Party Wall Act requires notice for excavation within 3 m (or 6 m) of neighbours' foundations, and surveyors record the condition of their property before work and check it after. Underpinning is done in short bays so walls are always supported.
Cellar conversions take eight to twelve weeks on site. Floor lowering and new basements take five to nine months, after two to four months of design, ground investigation, planning and party wall procedures.
On high-value plots where you cannot extend or convert the loft, often yes; the space is expensive but the alternative is moving. On a typical suburban semi with room to extend, an extension or loft usually gives better value. We say which at feasibility.
Read before you build
Basement Conversions
Before committing to a basement: ground and water table, existing head height, neighbours and party walls, the borough's basement policy, light and escape, and whether the numbers — £2,800 to £6,500+ per m² in 2026 — beat extending or moving.
7 October 2025 · 7 min read
Basement Conversions
Structural underpinning, waterproofing to BS 8102, drainage and pumps, insulation, light wells and escape, then the fit-out: what a basement conversion involves stage by stage and why it takes five to nine months.
21 October 2025 · 7 min read
Basement Conversions
Basement conversions have become one of the most sought-after home improvement projects in the UK, particularly in urban areas where space is limited and…
31 August 2025 · 6 min read
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