If you’re planning to build a basement beneath your property, it’s important to understand that it’s one of the most technically challenging forms of domestic construction — but also one of the most rewarding if done right.
We’ll take you through what the process typically involves, and how the Party Wall etc. Act 1996 plays a vital role in ensuring your project runs smoothly — and lawfully.
How Is a Basement Constructed?
The process usually begins with your architect, who will design the layout of the new basement: walls, stairs, storage areas, utility rooms and so on. After this, a structural engineer will design the load-bearing elements of the structure.
But one of the most important (and often overlooked) stages is when your builder or specialist contractor sets out how the work will be done safely. In fact, the contractor’s method statement — especially for excavation and underpinning — is crucial to avoid damage to your home and neighbouring properties.
What Is Underpinning?
Underpinning is the process of forming a new concrete wall directly beneath an existing structure, carried out in small, controlled stages. Rather than digging out large areas at once, the basement is formed section-by-section — this method supports the structure above while work is ongoing.
Here’s how it typically works:
- Small areas of soil beneath the existing foundations are carefully excavated
- Steel reinforcement is installed
- Concrete is poured into the space to form the underpin
- Once cured, adjacent sections are tackled in the same way, slowly forming a reinforced concrete ‘box’
Throughout the process, temporary props are used to prevent movement in the soil or existing walls. It’s slow, precise work — and that’s exactly why a competent contractor is essential.
What About Waterproofing?
Once the concrete structure is complete, it must be waterproofed internally. The most common method involves fitting a cavity drainage membrane — a durable, studded plastic sheet that lines the walls and floor.
Any water that seeps through the concrete is directed behind this membrane to a sump and pump system, which then drains to the sewer. The membrane is then hidden behind stud walls, giving you a dry, warm, finished space.
What Does the Party Wall Act Require?
In almost every basement project, formal party wall procedures are required before work begins.
Under the Party Wall etc. Act 1996, if you’re digging within 3 metres of a neighbouring property and to a depth lower than their foundations, you’re legally obliged to serve an Adjacent Excavation Notice under Section 6 of the Act.
This applies even if your property does not physically touch your neighbour’s — it’s about depth, proximity, and potential risk.
If your neighbour is proposing a basement themselves, you also have rights under the Act, and we can advise you on how best to respond.
Why You Should Involve a Party Wall Surveyor Early
Basements are complex projects — structurally, logistically, and legally. Serving notices and managing your obligations under the Party Wall Act is not something to leave until the last minute.
Engaging a Party Wall Surveyor early will help you:
- Understand your legal duties
- Prepare and serve the correct notices
- Avoid delays or disputes with neighbours
- Provide peace of mind to all parties involved
Planning a Basement? Let’s Talk.
At Simple Survey, we’ve guided hundreds of clients through the Party Wall process — from first-time homeowners to experienced developers.
📩 Email us at team@simplesurvey.co.uk to discuss your project or get free advice today.