The Extension Company is the extensions studio of Pine Grove Design & Build. We take on basements in North London first, and across London: converting cellars that are already there, lowering them, digging new lower-ground floors under period houses, and building basements beneath new rear extensions. In North London the work is designed, managed and built by Pine Grove's own team. That includes the parts many builders hand to a specialist subcontractor: the excavation, the underpinning, the waterproofing and the fit-out.
A basement is the most technical project most homeowners ever commission, and the most expensive per square metre. This page covers what the work involves, what each kind of basement typically costs, what Camden, Haringey, Barnet and Enfield each ask for, what those councils have decided lately, and the parts that go wrong when they are rushed.
What a basement project involves
Every basement goes through the same sequence, and the order matters.
- Survey and ground investigation. Trial pits expose the existing foundations, and boreholes show what the soil is and where the groundwater sits. Nothing below ground can be priced properly before this.
- Structural design. An engineer designs the underpinning, the retaining walls, the new slab and the temporary works that hold the house up while the ground beneath it is removed.
- Planning and party wall. The engineering evidence goes in with any planning application, and notices are served on every adjoining owner.
- Underpinning and excavation. The foundations are taken down to the new depth in short sections, in a set sequence, so no long stretch of wall is left unsupported. The ground is then dug out in stages behind them.
- The structural box. A reinforced concrete slab and walls form the new floor, tied into the underpinned foundations.
- Waterproofing and drainage. The box is made watertight, and any water that reaches it is collected and pumped away.
- Fit-out. Insulation, ventilation, electrics, plumbing, a staircase, fire protection and finishes turn the box into rooms.
Building Regulations apply throughout. An application is needed whether you are converting a cellar to habitable use, digging a new basement or extending one, and it covers structure, fire safety and means of escape, resistance to moisture, ventilation, drainage and energy performance.
Types of basement project, and what they cost
People use the word basement for four different jobs. They differ in price, in planning treatment and in risk.
| Type | What it is | Typical London cost |
|---|---|---|
| Cellar conversion | Waterproof, ventilate and fit out a cellar that already has usable head height. No dig and no underpinning | £1,500 – £3,000 per m² |
| Cellar with the floor lowered | Excavate inside an existing cellar and underpin the walls to gain head height | £4,500 – £7,500+ per m² |
| Full dig under the house | Excavate a new storey beneath the ground floor and underpin the whole footprint | £4,500 – £7,500+ per m² |
| Rear basement under or with an extension | Excavate at the back of the house, usually beneath a new ground-floor extension | £4,500 – £7,500+ per m² for the basement |
| Lightwell | An external excavation against the basement wall with glazing, for daylight and escape | Adds £15,000 – £25,000 |
The £4,500–£7,500+ per m² figure is a typical London market range in 2026 for a basement that has to be excavated: the dig, the underpinning, the waterproofing and the fit-out together. It is a guide to what basement work in London costs, not a quote, and it excludes VAT, professional fees and party wall costs. At that rate a 40 m² basement is £180,000–£300,000 and a 60 m² basement is £270,000–£450,000. Deep digs, wet ground and difficult access sit above the range, which is what the plus sign is for.
The lower band applies only where the space already exists at a usable height and nothing has to be dug or underpinned. The moment the floor has to come down, the job is an excavation and is priced as one. Our basement conversion cost guide sets out what moves a price within the range and the extras to budget for.
Cellar conversion
Many Victorian and Edwardian houses in North London were built with a coal cellar or a part-height undercroft. Where the head height is already usable and the walls are sound, converting it is a waterproofing and fit-out job: a drained membrane on the walls and floor, a sump and pump, insulation, ventilation, electrics, a staircase that meets the regulations and a finish.
Lowering the floor of a cellar
Most cellars are too low to stand up in. Gaining head height means digging below the existing foundations, so the walls have to be underpinned first. Structurally this is the same work as a new basement over a smaller area.
Full dig under the house
A new storey built beneath a house that stays standing. The foundations are underpinned all the way round, the ground is removed in stages, and a concrete box is cast to carry the building and keep the water out.
Rear basement under or with an extension
Where the garden allows, a basement can be dug at the back of the house with a ground-floor extension built over it, so one excavation produces two floors. The part beneath the garden is where councils look hardest: how much garden it takes, how much soil is kept above it, and what happens to trees. The extension on top is priced separately, as set out in our North London extension cost guide.
Lightwells
A basement used as a bedroom, living room or office needs daylight, ventilation and a way out in a fire, and a lightwell provides all three. It is also the part that changes how the house looks from outside, so it often decides whether a planning application is needed.
When a basement is the right answer
A basement is rarely the cheapest way to add a room. A rear extension, a side return or a loft conversion almost always costs less per square metre, and if your house can still go out or up, we will say so. A basement earns its place in three situations:
- There is nowhere else to go. The loft is done, the garden is too small to give up, and the side return is already built.
- The street is valuable enough. Where finished floor space sells for more per square metre than it costs to dig, the sums work. Where it does not, they rarely do.
- The outside of the house cannot change. In a conservation area, or on a house where a visible extension would be refused, a basement under the footprint leaves the street elevation as it is.
An existing cellar changes the sums. Converting space that is already there costs far less than digging new space, and can be worth doing on streets where a full dig would never pay.
Basement planning by council
Does a basement need planning permission?
It depends on the project and the council. National guidance is that converting an existing cellar into living space is unlikely to need planning permission, provided it stays part of the house and nothing changes outside. Digging a new basement commonly does need permission, and adding a lightwell usually settles the question because it alters the outside of the house.
Outside Camden, a basement built entirely beneath the footprint of a house, with no external change, can fall within permitted development, unless an Article 4 direction on your street has taken away the right to extend. It is not a safe assumption, because a large excavation can be treated as an engineering operation that needs permission in its own right. We would not rely on it without a lawful development certificate (£274), which puts the council's agreement in writing before any money goes into the ground. Flats and maisonettes have no permitted development rights to extend, so excavating beneath a flat, or changing the outside of one, always needs a planning application. For a house, the householder application fee is £548. A listed building needs listed building consent whatever the planning position.
Each council then applies its own basement policy. These are the four councils behind most of the North London areas we cover.
Camden: Hampstead, Belsize Park and the west side of Highgate
Camden is the strictest, and the clearest. An Article 4 direction covering the whole borough, confirmed on 3 October 2016 and in force since 1 June 2017, removed permitted development rights for basements, so every basement or lightwell excavation in Camden needs planning permission.
Applications are judged against Policy A5 of the Camden Local Plan, which limits how big a basement can be. A basement should:
- be no more than one storey deep, which Camden treats as about 3 to 4 metres of excavation
- not be built beneath an existing basement
- take up no more than 50% of each garden, with front, rear and side gardens counted separately
- be less than 1.5 times the footprint of the house in area
- extend into the garden no further than 50% of the depth of the house, and no further than 50% of the depth of the garden
- be set back from the neighbours' boundaries wherever it goes beyond the footprint of the house
Camden's guidance also expects at least a metre of soil above any part of a basement that extends beyond the house.
Every application needs a Basement Impact Assessment, and most are audited by an independent engineer at the applicant's expense. The assessment has to show that the risk of damage to neighbouring properties is no worse than Burland Scale 1, which means very slight. Camden will generally ask for a construction management plan as well.
Camden is replacing its Local Plan. The new plan, with a new basement policy numbered D6, went through examination hearings in May and June 2026, so we check which policy is in force on the day an application goes in. Our Hampstead and Highgate guides cover the local detail.
Haringey: Muswell Hill, Crouch End and the east side of Highgate
Haringey has no borough-wide direction like Camden's, and its policy accepts that some basement work falls within permitted development. Where an application is needed, it is judged against Policy DM18 of Haringey's Development Management policies, adopted in July 2017, which covers residential basements and light wells.
DM18 allows a new or extended basement only where it is shown that the work will not harm the structural stability of the house, its neighbours or the highway, will not increase flood risk, and will not harm the appearance of the property or the amenity of the people next door. Where a basement runs beneath the garden it must not damage trees of value, and it must keep enough soil depth for planting. Basements in areas prone to flooding cannot contain habitable rooms unless there is a reasonable means of escape.
That evidence goes in a Basement Impact Assessment submitted with the application, prepared by qualified independent assessors and paid for by the applicant. Light wells have a test of their own: the council looks at the character of the building and the street, and at whether the work would lose more than 50% of the front garden. See our guides to Muswell Hill and Crouch End.
Highgate: two councils and a neighbourhood plan
Highgate is split between Haringey and Camden, and both councils adopted the Highgate Neighbourhood Plan in 2017. Its Policy DH7 applies on both sides of the High Street. It says construction should not go ahead where the evidence shows damage to neighbouring properties would exceed Burland Scale 1, expects at least one metre of permeable soil above any part of a basement beneath a garden, and asks for a pumped device to protect against sewer flooding.
Barnet: Finchley, Golders Green, Hendon and Mill Hill
Barnet adopted a new Local Plan on 4 March 2025, and its Policy CDH06 covers basements and other below-ground development. A basement should be subordinate in size to the building it extends, protect trees and their roots on the site and on neighbouring land, keep the garden's amenity space, and be shown to suit the ground, taking account of land instability, contamination and groundwater. Rooms have to work as rooms, with adequate natural light and ventilation and the ceiling heights the London Plan sets. Railings, grilles and lightwells should not clutter the front of the house, and a site-specific flood risk assessment is required where appropriate.
As a general guideline, Barnet looks for at least half of the rear garden to stay free of development below ground and at its original level. In Hampstead Garden Suburb the Trust's consent is needed as well as the council's, as our Hampstead Garden Suburb Trust guide explains.
Enfield: Southgate, Winchmore Hill and Palmers Green
Enfield's Development Management Document, adopted in November 2014, has no separate basement policy. A basement is assessed under the borough's general design and extension policies, and the one specific requirement is on flooding: any proposal that creates usable space below ground in a groundwater flood risk area needs a site-specific flood risk assessment. A replacement Local Plan is going through examination, so the position may change. Far fewer basement applications are decided here than in Barnet or Haringey, as the borough table below shows.
Conservation areas and listed buildings
A basement under the footprint can suit a conservation area, because the street elevation does not change. The constraint moves to what can be seen: lightwells, railings and anything that alters a front garden. A listed building needs listed building consent for a new or extended basement even where planning permission is not required. Our guide to extending in a conservation area covers the wider rules.
Recent basement decisions in North London
Planning registers are public, so you can see what councils are deciding before you commit to a design. These are the 12 newest decisions that include basement work in the wards our North London area guides cover, decided by Barnet, Camden, Enfield, Hackney and Haringey. Each shows the street and postcode district only.
| Decided | Where | Outcome | What was applied for |
|---|---|---|---|
| 14 Sep 2026 | Dartmouth Park Avenue, NW5 Camden, Highgate ward | Approved | Excavation of basement including extension of front lightwells, erection of lower and upper ground floor rear extensions, erection of rear… Ref 2025/5526/P · Highgate area guide |
| 10 Sep 2026 | Lower Terrace, NW3 Camden, Hampstead Town ward | Approved | Demolition of existing garage and interlinking gallery and replacement with new garage and garden room with link, new basement floor… Ref 2025/1690/P · Hampstead area guide |
| 8 Sep 2026 | Gresham Avenue, N20 Barnet, Friern Barnet ward | Refused | Two storey side extension, single storey rear extension and extension and lowering of existing basement. New raised rear terrace… Ref 26/2265/HSE · Friern Barnet area guide |
| 21 Jul 2026 | Rosebery Road, N10 Haringey, Alexandra Park ward | Approved | Demolition of existing single storey rear extensions; erection of rear ground and basement level extensions; alteration to rear and side… Ref HGY/2026/0636 · Muswell Hill area guide |
| 7 Jul 2026 | Spencer Rise, NW5 Camden, Highgate ward | Approved | Extending basement area, extension to ground and first floor, rear dormer extension, insertion of conservation Velux windows, alterations… Ref 2026/1155/P · Highgate area guide |
| 30 Jun 2026 | The Mall, N14 Enfield, Arnos Grove ward | Approved | Single storey rear extension with green roof, enlargement of basement and internal reconfiguration. Ref 26/01741/HOU · Southgate area guide |
| 29 Jun 2026 | Leswin Road, N16 Hackney, Stoke Newington ward | Approved | Erection of a single storey wraparound rear extension; formation of a rear lightwell and front rooflight in connection with excavation of… Ref 2025/2657 · Stoke Newington area guide |
| 23 Jun 2026 | Harcourt Road, N22 Haringey, Alexandra Park ward | Approved | Demolition of existing rear timber terrace structure & stairs. Erection of two-storey rear ground floor and basement extension Ref HGY/2026/1226 · Muswell Hill area guide |
| 22 Jun 2026 | Firs Avenue, N10 Haringey, Muswell Hill ward | Approved | Erection of lower ground floor single storey rear extension; basement extension; rear dormer extension; associated internal and external… Ref HGY/2026/0923 · Muswell Hill area guide |
| 18 Jun 2026 | Dynevor Road, N16 Hackney, Stoke Newington ward | Approved | Lower existing basement floor and create front lightwell with glass walk on roof light Ref 2026/0421 · Stoke Newington area guide |
| 8 Jun 2026 | Lyndhurst Road, NW3 Camden, Hampstead Town ward | Approved | Basement excavation, boundary treatments, bin store, hard and soft landscaping and associated works. Ref 2025/5819/P · Hampstead area guide |
| 13 May 2026 | South Lodge Drive, N14 Enfield, Oakwood ward | Approved | Part single, part 2 storey rear extension and basement extension. Ref 26/00541/HOU · Oakwood area guide |
Across those wards, councils decided 35 applications that included basement work between 23 September 2025 and 22 September 2026: 30 approved and 5 refused. Refusals are part of the picture, and the reasons given for them are worth reading before you draw anything.
Barnet and Enfield references can be looked up on the Barnet Council planning register and the Enfield Council planning register.
Basement applications by borough
Borough-wide counts for Barnet, Haringey and Enfield, 23 September 2025 to 22 September 2026: every householder application, lawful development certificate and prior approval whose description mentions a basement or excavation. It is a wider net than the list above, so the two sets of figures are not the same count.
| Borough | Decided | Approved | Refused |
|---|---|---|---|
| Barnet | 24 | 16 | 8 |
| Haringey | 16 | 14 | 2 |
| Enfield | 3 | 2 | 1 |
Source: GLA Planning London Datahub, decisions to 22 September 2026. The datahub can lag a council's own register by a few weeks. Camden's feed carried no lawful development certificates in this window, so Camden decisions here are householder applications and prior approvals only. These are council decisions on applications from any applicant: they are not a list of our projects and not our approval rate.
The hard parts, explained honestly
The structural engineer and the impact assessment
A Basement Impact Assessment is the engineering case for the project. Camden's guidance sets out the stages: screening, scoping, site investigation and study, then the assessment of impact on land stability, groundwater and surface water. It has to be specific to your site, written by suitably qualified engineers and, in Camden, laid out so that the council's independent auditor can check it.
Two things follow. The ground investigation comes before the design, because the assessment cannot be written without it. And the assessment is a cost in its own right, separate from the build.
Waterproofing
Water is the thing that ruins basements, and when it gets in, the cause is usually design or workmanship, not bad luck. BS 8102, the British Standard for below-ground waterproofing, sets out three approaches: a barrier applied to the structure (tanking), a structure that is watertight in itself (waterproof concrete), and a drained cavity that lets any water through to a channel, a sump and a pump. A habitable basement normally combines two of them, so that one failure does not mean a wet room.
Drained systems depend on pumps, and pumps need power, servicing and a backup, so we design the sump, the pumps, the alarm and the access for maintenance as part of the structure. Basements are also exposed to sewer flooding, which is why Camden's guidance and the Highgate Neighbourhood Plan both ask for a pumped device on the drainage.
Party wall
Almost every basement triggers the Party Wall etc. Act 1996, and usually more than one part of it. Excavating within 3 metres of a neighbour's building, to a level below their foundations, needs a notice under section 6 at least one month before work starts. So does excavating within 6 metres where the dig would cut a 45-degree line drawn down from their foundations. Underpinning a wall you share is work to the party structure itself, which needs a notice at least two months ahead.
A neighbour who does not consent in writing within 14 days is treated as having dissented, and surveyors then agree an award that records the condition of the neighbouring house and sets out how the work will be done. The owner doing the work pays: budget £1,000–£2,500 per affected neighbour where surveyors are appointed, and more for a complex basement award. Our Party Wall Act guide explains the process, and the guide to what each project triggers sets out which notice applies when.
Neighbours and the street
A basement is months of excavation beside other people's homes. Haringey's policy requires that construction does not harm road safety or place unreasonable inconvenience on people nearby, and that noise, vibration and dust are kept to a minimum. Camden generally requires a construction management plan covering traffic, parking, noise and the stability of neighbouring buildings. In practice, neighbours should hear about the project from you before they hear from a surveyor, and spoil removal needs a plan that works on your street.
Timescales
A cellar conversion with no dig typically takes 8 to 16 weeks on site. A new excavation is 6 to 12 months of construction. Before that come the ground investigation, the engineering, the planning application and the party wall awards, which is why a new basement usually runs 12 to 18 months from first visit to handover.
Planning is the least predictable part. A householder application has an eight-week target, but a basement application with an audited impact assessment often takes longer. Among the Camden approvals in our data for the twelve months to 22 September 2026, two carry reference numbers issued in 2021 and 2023 and were only decided in 2026. We run the party wall process alongside planning, not after it, so the two finish together.
How we work
- Free site visit. We look at the house, the cellar if there is one, the access and the street, and tell you whether a basement is the right project or whether an extension or loft would give you more for the money.
- Investigation and design. Ground investigation first, then the architectural and structural design. You get a fixed, itemised price once the ground is understood, not a guess made before it.
- Planning and party wall. We submit the application with the impact assessment the council requires, and serve the party wall notices while it is being decided.
- The build. In North London, Pine Grove's own team carries out the excavation, the underpinning, the waterproofing and the fit-out.
- Sign-off. Building control completion, and a handover of the waterproofing and pump details with how to look after them.
Basement guides
- Basement conversion cost in London: the per m² range, what moves it, and the extras to budget for
- Cellar conversion or new dig? How the two projects differ in planning, party wall and time
- The Party Wall Act guide: notices, surveyors, awards and costs
- House extensions in North London: how Enfield, Barnet, Haringey and Camden compare, borough by borough
- Lawful development certificates, for a basement that stays within permitted development
- Extension planning fees: what each type of application costs in 2026
We build basements in North London and take on basement projects across London. Every area we cover is on the locations page.