If you are extending a terraced or semi-detached house in London, there is a very high chance the Party Wall etc. Act 1996 applies to you. It is not planning permission. It is not building regulations. It is a separate legal process that runs alongside both, and it exists to protect your neighbours from damage caused by your building work.
Most homeowners find out about it late, usually when a builder mentions it a fortnight before they want to start. By then it is already a delay. This guide covers what triggers it, what it costs, how long it takes and what to do if a neighbour digs their heels in.
What the Party Wall Act actually covers
Three types of work fall under the Act. If your project involves any of them, you have a legal duty to notify the affected neighbours before you start.
Work on an existing party wall. This is the wall you share with the house next door. Cutting into it to insert a steel beam, removing a chimney breast, raising it, underpinning it or rebuilding part of it all count. On a rear extension where steels bear onto the party wall, this is almost always triggered.
Building on the boundary line. If you intend to build a new wall up to or astride the boundary with your neighbour, you must notify them. This catches most side return extensions and wraparound extensions, where the new wall sits right on the line.
Excavation near a neighbouring structure. If you are digging foundations within three metres of a neighbouring building and going deeper than their foundations, notice is required. Within six metres, a different test applies based on the angle of the excavation. Basement projects trigger this without exception.
The short version: if your extension touches a shared wall, sits on a boundary, or involves digging deeper than next door's foundations, you need to serve notice. In practice that covers the large majority of London extensions.
What it does not cover
Plenty of work sits outside the Act. Internal alterations that do not touch a party wall, a loft conversion that stays entirely within your own roof structure and does not bear on the shared wall, replacing windows, or a detached garden building well away from the boundary. If you are unsure, the test is whether the work could realistically affect the structure your neighbour relies on.
The notice period, and why it decides your start date
The notice period depends on the type of work.
Two months for work to an existing party wall. One month for excavation near a neighbouring structure, and for building a new wall on the boundary.
That clock starts when the notice is served, not when you post it or when the neighbour gets round to reading it. And you cannot start work before it expires unless every affected neighbour gives written consent.
This is the single most common cause of avoidable delay on a London extension. If notices go out two weeks before the intended start, you have just added two months to the programme. Served early, in parallel with the planning application, the period runs down while you are waiting for planning anyway and costs you nothing at all.
What happens after you serve notice
Your neighbour has fourteen days to respond, and there are three possible outcomes.
They consent in writing. Best case. No surveyor, no award, no fee. You can start once the notice period expires. Roughly a third of London notices end here, usually where the relationship is good and the work is modest.
They dissent and appoint their own surveyor. You each have a surveyor, the two agree a Party Wall Award, and you pay for both. This is the most expensive route.
They dissent and agree to an Agreed Surveyor. One surveyor acts impartially for both parties. Cheaper and usually faster. Worth proposing explicitly in your covering letter, because many neighbours do not know it is an option.
They do not respond at all. After fourteen days of silence, a dispute is deemed to have arisen. You can then appoint a surveyor on their behalf. It sounds adversarial but it is the mechanism the Act provides to stop a project being blocked by someone simply ignoring the post.
What a Party Wall Award contains
The Award is the document that governs the work. It sets out what you are permitted to do, working hours, access arrangements, and how any damage will be dealt with.
The most useful part is the Schedule of Condition. This is a photographic record of your neighbour's property before work starts, and it protects both sides. If a crack appears afterwards, the schedule settles whether it was there beforehand. Without one, you are arguing about it from memory, and that argument rarely ends well.
What it costs in London
Party wall costs are separate from your construction budget and separate from professional design fees. Budget for them from the outset.
A straightforward notice served with neighbour consent costs nothing beyond your own time or a small fee if a surveyor drafts it, typically £150 to £400.
An Agreed Surveyor handling one neighbour usually costs £900 to £1,600 in London.
Two surveyors, where the neighbour appoints their own, typically comes to £1,800 to £3,500 in total, because you pay both.
A mid-terrace house with neighbours on both sides can double these figures, since each adjoining owner is a separate party. Basement projects sit at the top of the range because the technical scrutiny is far greater.
Common mistakes
Serving notice too late. Covered above, and by far the most expensive error in terms of programme.
Assuming planning permission covers it. It does not. They are entirely separate regimes. You can have full planning permission and still be acting unlawfully if you start work without a valid party wall award.
Getting the notice wrong. An invalid notice is no notice. It must identify the correct adjoining owners, describe the works accurately and include the right dates. If ownership is unclear, for example a flat with a freeholder and a leaseholder, both may need serving.
Skipping it because relations are good. Relationships change, and houses get sold. The Schedule of Condition protects you as much as your neighbour. If a new owner moves in and blames your extension for a pre-existing crack, that photographic record is the only thing standing between you and a bill.
How we handle it
Because we design and build under one contract, party wall timing gets built into the programme rather than bolted on. We identify at feasibility stage which neighbours need notifying and under which section of the Act, then serve notices early so the period runs alongside the planning application instead of after it.
We do not act as party wall surveyors, since the role has to be independent. But we coordinate with surveyors we work with regularly, and we make sure the construction sequence matches what the Award actually permits. Plenty of disputes come from a builder working outside the agreed hours or accessing land the Award does not cover, and that is entirely avoidable.
If you are weighing up an extension and want to understand the full cost picture including the parts people forget, our cost calculator gives you a range in under a minute, and our extension cost guide breaks down where the money actually goes.