Basement Conversions: The Notice That Governs the Job

A basement is governed by a notice, not by a design. Dig within 3 metres of a neighbour's building and below the bottom of their foundations and section 6 of the Party Wall etc. Act 1996 applies — and section 6(3) lets them require you to underpin their house at your own expense. That is a statutory right, exercised at your cost, and it is the reason a basement's structural budget is not entirely yours to control.

Most basement advice starts with the design and gets to the neighbours somewhere near the end. On site it runs the other way round. The excavation notice decides when you can start, what you have to safeguard, and who else gets a say — and almost all of that is settled by geometry rather than by anything you choose.

The two triggers, and the geometry behind them

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.

Read the two tests carefully, because both limbs have to be satisfied on each. Three metres away but not going below their foundations is not caught by 6(1). Going far deeper but eight metres away is caught by neither. It is the combination of proximity and depth that triggers the duty — which is why the answer comes from a section drawing rather than from a plan.

The subsection that decides your structural budget

Section 6(3) is a right your neighbour holds and you fund. They may require you to underpin or otherwise safeguard their foundations, and you pay for it, "so far as may be necessary". You cannot decline it and you cannot price it confidently before you know whether it will be asked for.

So a basement quotation given before notices are served contains an unpriced variable that is not the builder's to remove. Anyone presenting one as a settled figure is either not aware of 6(3) or is hoping you are not.

Silence is not consent, and it costs you a month

Section 6(7) is the provision that catches the most people, because it inverts the ordinary intuition. A neighbour who says nothing has not tacitly agreed. At day fifteen they are deemed to have dissented, and a dispute is deemed to have arisen — the same statutory position as a flat refusal, with the same consequence of surveyors and an award.

That is not a disaster. It is the ordinary route, and what a party wall agreement costs and how the process runs sets out the surveyor appointments, the award and the timings. But it needs to be in the programme from the start rather than discovered on day fifteen, because the alternative is a project that assumed consent and budgeted for none of it.

Do not serve early to get the clock running

The instinct is to serve as soon as possible. Section 6(8) punishes it: the notice lapses if work has not begun within twelve months and is not prosecuted with due diligence.

Two things follow. First, the notice cannot be served until the design shows depth, because 6(6) requires plans and sections showing the site and depth of the excavation — so the party wall clock genuinely starts later than most programmes assume. Second, if planning is likely to be slow or contested, serving early risks reaching twelve months without a start and beginning the whole process again.

Sequence it: design to the point where depth is fixed, resolve planning risk, then serve.

The structural work itself

Underpinning is what most basement projects are actually buying, and it is priced by sequence rather than by volume — short designed bays, poured one at a time, each waiting on the last. Why underpinning costs what it does sets out that queue, which is the single largest driver of a basement programme and the reason the work cannot simply be accelerated with more people.

Add to that: the excavation safety duties under CDM, temporary works design, waterproofing to a defined grade with a maintainable drainage route, and the knowledge that everything removed leaves through the house or through a hole in the front garden.

Planning is decided by your borough, not by London

Permitted development rights for basements have been removed in parts of London. Hammersmith and Fulham removed them borough-wide by Article 4 Direction in force from 26 April 2018 — so in that borough, planning permission is required. Several other boroughs operate basement-specific policies on depth, on how far a basement may extend under a garden, and on construction management plans.

There is no London-wide answer, and a neighbour's completed basement proves nothing about yours: it may predate a direction, or sit in a different borough two streets away.

Before you commission a design

1. 01Measure the horizontal distance to every neighbouring structure. Not just the houses either side. Section 6 says any part of a building or structure of an adjoining owner — a garage, an outbuilding, a substantial garden wall. Three metres and six metres, measured horizontally, from each.
2. 02Find out where their foundations bottom out. Both section 6 tests depend on it, and neither can be answered from a plan. This is a section drawing and, usually, a trial hole. Until you have it you do not know whether you owe a notice at all.
3. 03Budget for section 6(3) as a range, not a line. Your neighbour may require underpinning of their foundations at your expense. Treat it as a provisional sum with a stated basis rather than a number, and be suspicious of any quote that omits it entirely.
4. 04Check your borough's Article 4 position for your own address. Permitted development for basements is removed in parts of London. Borough by borough, address by address, and before any design fee is spent.
5. 05Sequence the notice after depth is fixed and planning risk is resolved. Section 6(6) needs plans and sections showing depth, and section 6(8) expires the notice after twelve months without a start. Serving early to feel productive can cost you the whole process twice.
6. 06Ask what leaves the site, and how. Spoil from a London basement goes out through the house or through the front garden, in loads sized by the street rather than by efficiency. That constraint prices the job as much as the digging does.

Why a builder is writing this, and what we are not saying

There is no price on this page. Our estimator covers lofts, extensions, renovations and new builds, and a basement is none of them — so any figure here would be typed rather than calculated, and a basement is exactly the project where a typed figure does the most damage. For an extension or a loft, the estimator will give you a real band.

What we will say is that a basement is the project with the widest gap between the drawing and the outcome, and almost all of that gap is other people's decisions: your neighbour's rights under 6(3), your borough's Article 4 position, and what the ground actually turns out to be.

2VP is the builder, not a marketplace: one contract, one named project lead, and no introducer fee stacked on the build cost. On a job with this many parties, the question of who is accountable when the ground disagrees with the drawing is not an abstract one.

The one line to take away

Get a section drawing showing your neighbours' foundation depths before you spend anything on design. It is the drawing that tells you whether section 6 applies, what you may be required to underpin, and when your clock actually starts.