Surface Water Soakaway Design for Extensions

Surface Water Soakaway Design for House Extensions in South-West Scotland & Cumbria

A new extension adds roof and hardstanding area that the existing drainage often can't take — which is why Building Control/Building Standards, or a condition on your planning permission, will usually ask you to show how the extra surface water is being disposed of on site. I carry out the percolation test and prepare a full soakaway design — backed by 45 years' experience in Building Control and Building Standards, including senior local authority roles.

Get a Quote How It Works

Site visit required — currently covering Dumfries & Galloway, the Scottish Borders and Cumbria only

Also carry out foul drainage soakaway design — see the full percolation testing & soakaway service

Why Extensions Need This

A new roof and hardstanding area increases the amount of surface water your site sheds — get the soakaway wrong and you risk a failed inspection, a resubmission, or flooding at the boundary.

Often a Planning or Building Control/Standards Condition

Many extension approvals carry a condition requiring surface water to be disposed of on site rather than added to an already-stretched sewer or ditch. A percolation test and soakaway design is how you discharge it.

Tested to Recognised Standards

Percolation testing carried out in line with BRE Digest 365, giving Building Control, Building Standards, SEPA and the Environment Agency a result they can rely on.

45 Years' Regulatory Insight

Having spent decades assessing these very submissions from the other side of the desk, I know exactly what a drainage design needs to demonstrate to pass first time.

Test and Design in One Visit

No need to instruct separate contractors — the percolation test, soakaway sizing calculations and drainage design report are all prepared by the same Chartered Surveyor.

Ongoing Advisory Support

If Building Control, Building Standards, SEPA or the Environment Agency come back with queries once you've submitted, I'm on hand to help you respond — and happy to liaise directly with your architect, engineer or drainage designer if you have one acting on your behalf.

How It Works

A straightforward process — you provide access and a source of water, I do the rest.

1

You Dig the Trial Hole

I'll confirm the size and depth of the trial hole needed near your extension. You (or your groundworker) dig this before my visit and provide a source of water on site.

2

Site Visit & Percolation Test

I attend site, carry out the percolation test to BRE Digest 365, and record ground conditions at the trial hole.

3

Soakaway Design & Report

I calculate the soakaway size needed for your extension's additional roof and hardstanding area and prepare a full surface water drainage design report, ready for submission to Building Control, Building Standards, SEPA or the Environment Agency, as applicable.

4

Advisory Support

Available afterwards to advise on queries from the authority, or changes needed if ground conditions vary once construction starts. If you have an architect, engineer or drainage designer acting on your behalf, I'm happy to liaise with them directly.

Coverage Area

Because this service requires a site visit, I'm currently only able to offer it within a limited travelling distance — the same area I cover for full percolation testing.

Guide Pricing

A surface water soakaway for an extension is a more straightforward job than a full foul drainage design — just the one trial hole — so the pricing is simple too.

From £246
Within 30 miles of Lochmaben
  • Single trial hole percolation test
  • Soakaway design & report included
  • Fixed price confirmed before booking

Further afield? That's not a problem — get in touch with your postcode and I'll confirm a fixed price, including any travel, before any work begins.

Frequently Asked Questions

Do I definitely need a soakaway for my extension?

Not always — it depends on the size of the extension and what your existing drainage can take. But where the roof and hardstanding area increases significantly, or your planning permission or Building Control/Building Standards approval carries a drainage condition, on-site infiltration is usually required. Get in touch with the extension size and your local authority reference if you have one, and I'll advise honestly.

Do I need to dig the trial hole myself?

Yes — you or your groundworker digs the trial hole and provides a source of water on site before my visit. I'll confirm exactly the size and depth once I know a little about your site.

Is this suitable for Building Control, Building Standards, SEPA and Environment Agency submissions?

Yes. The percolation test and drainage design are prepared to BRE Digest 365, which is the recognised methodology Building Control and Building Standards expect to see, along with SEPA in Scotland and the Environment Agency in England (including Cumbria).

Do you liaise with my architect or drainage designer?

Yes — if you have an architect, engineer or other designer acting on your behalf, I'm happy to liaise with them directly to make sure the drainage design ties in with the wider scheme.

Do you cover areas outside Dumfries & Galloway, the Borders and Cumbria?

Possibly, for sites reasonably close by — get in touch with your postcode and I'll let you know honestly whether it's practical for me to attend.

Get a Quote

Tell me a little about your extension — location, size, and ground type if known — and I'll come back with a fixed price and available dates.

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Or use the enquiry form on the main ThermaCheck site.

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