Drainage design is the part of every extension, basement, loft and new-build that decides where rainwater and waste actually go. Get it wrong — or skip it — and you’re looking at flooded patios, planning refusals, or sewage backing up into the kitchen.
That’s where a Structural Engineer Near Me comes in.
Let’s keep it simple and useful.
A Structural Engineer West London takes your property or project and works out — mathematically — exactly where every litre of water and waste needs to go, and how to get it there safely.
That package is what stands between a clean build and a flooded garden, refused planning, or a Thames Water enforcement letter.
Call 0207 101 3687 and receive a no obligations quotation
We’ll arrange a visit that suits you, walk the property, locate existing manholes and gullies, and gather everything the drainage design needs.
After the inspection, we’ll run the calculations, produce drainage layouts and specifications for build and Building Control approval.
Throughout the project, you’ll always be able to reach us for any assistance — whether it’s builder queries, site changes or Building Control.
This is where most homeowners get confused — so let’s break it down clearly. Drainage isn’t just “running a pipe to the sewer”; it’s a sized, calculated system tied to rainfall, ground conditions, planning policy and the public sewer network.
Every property already has a drainage story — manholes, gullies, soakaways, downpipes. We map what’s there before designing anything new.
The survey tells us:
Once the layout is mapped, we calculate the actual numbers behind it:
Every pipe, gully, manhole and soakaway is sized to BS EN 752, BS EN 12056 and Building Regulations Part H. No guesswork, no “that’ll do”.
London Clay, high water table, conservation paving, build-over of public sewers — every West London site has its own drainage quirks. A good design reflects the real site, not a textbook one.
Not all drainage design is the same — the right system depends on what you’re building, what’s already there, and what your local water company and planning authority will accept.
The bread and butter of domestic drainage. Used for any extension, new paving, garden re-landscape or roof alteration that changes run-off.
Used wherever new kitchens, bathrooms, utility rooms or basement WCs are added — and connection to the public sewer is involved.
Now required by most West London planning authorities for any sizeable extension or new build — surface water has to be managed on site, not pushed straight into the sewer.
Used where the ground can absorb run-off — but only after a percolation test proves it. Standard practice in parts of West London with sand, gravel or river-terrace deposits.
If your property is in:
You’re likely dealing with:
Any of these factors can quietly turn a simple drainage job into a more careful one. A local chartered structural engineer knows which assumptions are safe, which need verification, and how to keep planning, Building Control and Thames Water onside.
Not all engineers are equal — especially for drainage on older West London stock and tight-site extensions.
Yes — any extension, basement, paving project or new bathroom that changes run-off or foul flows should have a chartered engineer’s calculated drainage design. Building Control and the water company will both expect it.
Sometimes, for the very simplest like-for-like replacement. But for anything more — extensions, new bathrooms, paved areas, basements, or anything near a public sewer — a designed drainage layout is essential.
Typical domestic drainage design runs 1–3 weeks from instruction to issued drawings and calculations, assuming the site visit and any percolation testing go smoothly.
Almost always, yes. Drawings lie, manholes don’t. We need to lift covers, check inverts, falls and pipe materials, and locate any existing soakaways or build-overs before we design — otherwise the new system rests on assumptions.
Often, yes. Most West London boroughs now require a Drainage Strategy and SuDS pro-forma with the planning application for any sizeable extension or new build. Skipping it is a fast route to a refused application.
London Clay and overloaded combined sewers. Soakaways often won’t work, and the existing sewer is sometimes already running near capacity. The safest design tests the ground, checks Thames Water capacity, and uses SuDS attenuation where infiltration isn’t viable.
Yes — if you’re building within 3m of a public sewer, Thames Water build-over consent is normally required. Your engineer should flag this at the design stage and prepare the application alongside the drainage drawings.
For older properties, basement projects or anything connecting to existing drains — yes, often. A drainage survey or CCTV survey of the existing system feeds real data into the design and avoids nasty surprises mid-build.
