Free to size. Building Control-ready PDF £25 +VAT (£30 inc VAT) — no engineer needed.
Runs in your browser. Enter your percolation test and the area draining to the soakaway.
Why an extension triggers a soakaway
A new roof and any new paving add surface water that has to go somewhere. Under Building Regulations Approved Document H3, the first choice for that rainwater is to soak it into the ground — and since you usually can't load the extra runoff onto an existing drain or the public sewer, Building Control will typically ask for a soakaway designed to BRE Digest 365. That's exactly what SoakMonkey produces.
What you'll need
Two things: a percolation test (three fills in a trial pit, so the tool can work out how fast your soil drains) and the area of new roof and paving draining to the soakaway. Enter those and SoakMonkey runs the BRE 365 storm analysis and returns the size that passes.
Do it yourself — no engineer, no specialist software
You don't need a drainage consultant for a straightforward extension soakaway. SoakMonkey does the full BRE Digest 365 method — infiltration rate, design rainfall with a climate-change uplift, and the half-drain check — and gives you a branded, Building Control-ready PDF. Free to size on screen; £25 +VAT for the report. Prefer to hand it over? Just ask us.
Which soakaway suits an extension?
On a tight extension plot, a crate soakaway gives the most storage in the smallest hole; a traditional rubble pit is simpler but needs a bigger dig. SoakMonkey sizes all four constructions so you can pick what fits.
Frequently asked questions
Why does my extension need a soakaway?
Building Regulations (Approved Document H3) require the rainwater from a new roof or paved area to be disposed of properly, and the preferred route is to soak it into the ground. Because you usually can't just add the extra runoff to an existing drain or the public sewer, Building Control will normally ask for a soakaway designed to BRE Digest 365.
Do I need a percolation test for the soakaway?
Yes. The size of the soakaway depends on how fast your ground drains, and the only way to know that is a percolation (soakage) test — three fills in a trial pit. SoakMonkey works out the soil infiltration rate from your three fills, exactly as BRE 365 requires.
How big does an extension soakaway need to be?
There is no fixed size — it depends on your soil's infiltration rate, the area of new roof and paving draining to it, and the local rainfall. A slow-draining clay site needs far more storage than free-draining sand. SoakMonkey sizes it properly for your figures rather than guessing.
Can I do this myself, or do I need an engineer?
You can do it yourself. Run the percolation test, enter the results and the area draining to the soakaway, and SoakMonkey produces a Building Control-ready design to BRE Digest 365. The tool is free to use; the PDF report is £25 +VAT (£30 inc VAT). If you'd rather hand it over, we're happy to help.
Ready? Open the soakaway calculator, start with the percolation test, or read the BRE Digest 365 method.