What drawings do I need for a basement conversion in London?

Yes, you can get plans for a basement conversion, and what they contain depends first on whether you are fitting out a cellar that already exists or digging a new one. Our basement conversion drawings start at £1,995 for an existing cellar, and a new dig usually adds a planning set, a structural engineer's design and party wall notices on top of the building regulations drawings every conversion needs.

By the DrawingsDirect drawing team · 31 October 2026 · 7 min read

Existing cellar or new basement: the question that sets the drawing list

Most of the confusion about basement drawings comes from treating two different projects as one. An existing cellar already has walls, a floor and a stair; the work is to make it dry, warm, ventilated and safe to escape from, so the drawings are aimed mainly at building control. A new basement, or a cellar whose floor is dug down to get head height, removes soil from under a lived-in house. That is a structural project first, and it usually pulls in planning, an engineer and the neighbours.

We cannot tell you which category your house falls into from a description alone. A cellar that looks usable can turn out to need its floor lowered by the time a finished floor, insulation and a waterproofing layer go in, which moves the job into the second group. That is why many London clients start with our basement feasibility report at £695, which measures the head height and sets out the likely route before anyone pays for a full drawing set.

How a London basement drawing set comes together

Measure the cellar
Head height, wall and floor condition, the stair, the drains and the levels against the street and the garden.
Check the borough
The council's basement policy, any Article 4 direction, listed status and conservation area, before a design is fixed.
Bring in the engineer
Where the floor drops or the footprint grows, a structural engineer designs the underpinning, slab and retaining walls.
Draw both sets
Planning plans and sections where needed, then regs drawings with escape, waterproofing, ventilation and insulation.

What the building regulations drawings show for a basement

Every basement turned into a habitable room needs a building regulations set, whether the cellar is old or the hole is new. The detail that most often decides the layout is fire escape. Approved Document B says basement storeys containing habitable rooms should have either an emergency escape window or external door, or a protected stairway. In practice that means a light well with an escape window, a door out to a lowered garden, or a stair enclosed so it leads safely to a final exit. We show the chosen route on the plans and the section, because it shapes where the bedroom or office can go.

Water is the other big subject. Approved Document C points designers to BS 8102, which includes recommendations for floors subject to water pressure, the code of practice for protecting structures against water from the ground. Our drawings show where the waterproofing goes and how it meets the walls, floor and drainage, but the system itself (a cavity drain membrane with a sump, a tanking layer, or both) is normally designed and warranted by a specialist waterproofing contractor. The regs set also covers insulation, mechanical ventilation and, where the floor is lowered, the stair and head height.

Metal stud partitions and stacked plasterboard in a room mid-conversion, similar to a basement being lined before fit-out — Lining a basement is the last step: the waterproofing and insulation behind it are what the regs drawings record.

Planning: why Kensington and Chelsea, Camden and Westminster read basements differently

Fitting out an existing cellar with no outside change is often not something the council needs to see. Digging out, adding a light well or extending under the garden usually is, and three central boroughs have written basement policies of their own. Kensington and Chelsea's Policy CL7 says a basement should not comprise more than one storey, with exceptions only on large comprehensively planned sites, and should not exceed a maximum of 50% of each garden. The same policy rules out excavation underneath a listed building.

Camden's Policy A5 sets a similar ceiling. Its adopted Local Plan says basement development should not comprise of more than one storey, should not be built under an existing basement and should not exceed 50% of each garden within the property. An earlier revision of Westminster's basement policy, published through its council committees, said a basement should not involve digging out more than one storey beneath the original floors of the house unless exceptional circumstances are shown. Councils revise these plans, so we read the current version for your street before we design. Our basement planning permission guide explains the permitted development side in more depth.

Three-storey stock brick Victorian villas with bay windows, a London house type where front light wells often serve basements — On many Victorian streets a front light well is the visible part of a basement, and the council reads it as a change.

The structural engineer's part, and where our drawings stop

We are a chartered architectural technologist practice, so we draw the layout, sections, elevations and building regulations details. We do not design the structure that holds a house up while soil comes out from under it. Underpinning sequences, the new slab, retaining walls, steel beams and the temporary works during the dig are the structural engineer's design, and their calculations sit alongside our drawings in the building control submission.

Councils with basement policies usually want that engineering thinking before they decide. That Westminster revision asked for a detailed structural methodology statement and self-certification by a suitably qualified engineer with every basement application, and said the statement should be prepared and certified by a chartered civil or structural engineer. Camden's policy asks for a Basement Impact Assessment. We coordinate our planning drawings with the engineer's scheme so the two tell the same story. If you want to see how the engineering side is priced, our post on what structural calculations cost sets it out.

Two steel I-beams lying on gravel with paper labels attached, the kind of section an engineer sizes for a basement opening — Beams, underpinning and retaining walls are sized by the structural engineer, not on our drawings.

Party wall notices: the three and six metre rules

Almost every London basement dig touches the Party Wall etc. Act 1996, because terraced and semi-detached houses sit close to each other. Section 6 applies where you excavate within a distance of three metres measured horizontally from a neighbour's building and go lower than the bottom of its foundations. It also applies within six metres where the excavation would meet a plane drawn downwards from the bottom of the neighbour's foundations at 45 degrees.

The same section says the building owner must, at least one month before beginning to excavate, serve on the adjoining owner a notice indicating the proposals, with plans and sections showing the excavation. Our drawings are often what goes with that notice. Whether the neighbour agrees or a surveyor is appointed is outside our control, so the timetable should allow for it. Our post on party wall notice costs explains the notice and surveyor stages.

What basement drawings cost, and what sits outside the fee

Our basement conversion drawings start at £1,995 for an existing cellar, £2,395 where the floor is lowered and £2,895 for a new basement or light well. The fee is fixed before we start and covers our drawings. The structural engineer's design, the waterproofing specialist, the party wall surveyors, any Basement Impact Assessment, the council's planning fee and the building control charge are separate, and on a new dig they can add up to more than the drawings.

If you are not yet sure the project is viable, the feasibility report at £695 is the lower-risk first step. For the whole budget picture beyond drawings, our basement conversion cost guide covers build costs, and the basement conversion drawings page shows exactly what each drawing package includes.

Our basement drawing fees in London, by project type

Basement feasibility report: £695
Existing cellar conversion drawings, from: £1,995
Lowered floor basement drawings, from: £2,395
New basement or light well drawings, from: £2,895
Total: £7,980

Sources

Every figure in this article, where it comes from and a link to the original.

para 2.16: Approved Document B Vol 1 (2026 PDF) para 2.16: (source wording: "Basement storeys containing habitable rooms should have one of the following") an emergency escape window or external door, or a protected stairway. assets.publishing.service.gov.uk
BS 8102: Approved Document C (2013) para 4.12: (source wording: "BS 8102:1990 includes recommendations for floors subject to water pressure"). assets.publishing.service.gov.uk
1: RBKC Policy CL7(b): (source wording: "not comprise more than one storey. Exceptions may be made on large comprehensively planned sites"). planningconsult.rbkc.gov.uk
50%: RBKC Policy CL7(a): (source wording: "not exceed a maximum of 50% of each garden"). planningconsult.rbkc.gov.uk
1: Camden Local Plan Policy A5(f)-(h): (source wording: "not comprise of more than one storey; g. not be built under an existing basement; h. not exceed 50% of each garden within the property"). camdocs.camden.gov.uk
1: Westminster basement revision (main modifications): a basement should (source wording: "not involve the excavation of more than one storey") below the original floor levels unless exceptional circumstances are demonstrated. committees.westminster.gov.uk
statement: Westminster basement revision: all applications accompanied by (source wording: "A detailed structural methodology statement and appropriate self-certification by a suitably qualified engineer"), prepared and certified by a Chartered Civil Engineer (MICE) or Structural Engineer (MIStructE). committees.westminster.gov.uk
3: Party Wall etc. Act 1996 s6(1): excavation (source wording: "within a distance of three metres measured horizontally") of an adjoining owner's building, below the bottom of its foundations. www.legislation.gov.uk
6: Party Wall etc. Act 1996 s6(2): excavation within six metres meeting a plane drawn downwards (source wording: "at an angle of forty-five degrees to the horizontal"). www.legislation.gov.uk
1: Party Wall etc. Act 1996 s6(5)-(6): (source wording: "at least one month before beginning to excavate") serve notice, accompanied by plans and sections showing the site and depth of the excavation. www.legislation.gov.uk

Questions people ask

Can I get plans for a basement conversion without an engineer?

For an existing cellar with no structural change, sometimes. Once the floor is lowered, walls are underpinned or the footprint grows, a structural engineer has to design that work, and building control will expect their calculations.

Does converting an existing cellar need planning permission?

Often not, if nothing changes outside and the use stays part of the house. A new light well, a lowered floor, a listed building or a council Article 4 direction can change that, so we check before you commit.

How deep can a basement be in Kensington and Chelsea or Camden?

Both boroughs' adopted policies say a basement should not comprise more than one storey, with narrow exceptions, and should not take more than half of each garden. The exact wording is in each council's own plan.

Who designs the basement waterproofing?

Usually a specialist waterproofing designer or contractor, working to BS 8102. Our drawings show where the system goes and how it meets the structure and drainage.

Do I need a party wall notice for a basement?

Usually, if you excavate within three metres of a neighbour's building and below its foundations, or within six metres in the cases section 6 of the Party Wall etc. Act 1996 describes. The notice goes at least one month before digging.

Photographs: Rohan Gangopadhyay (Unsplash License); Olek Buzunov (Unsplash License); T (Unsplash License); W.carter (CC0).

Basement Conversion Drawings: a fixed fee from £1,995

Read more on the basement conversion drawings service page, or send your address below and we reply within one working day.

Basement Conversion Drawings

from £1,995

£1,795 drawings + £200 survey (£1,995 total)

Every price is the full fixed fee you pay. No VAT is charged. Council application fees are paid to the council separately.

Get a fixed quote Call 020 7864 7245

What you get

A measured survey of the cellar, head height, stair and levels
A check of your borough's basement policy before design
Planning plans, sections and elevations where the council needs them
Building regs drawings with escape route, waterproofing and ventilation
Drawings coordinated with your structural engineer's scheme
Plans and sections ready to go with a party wall notice