Free to work out. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Enter your percolation test and the area draining to the soakaway.
It depends on three things
A soakaway's size is driven by how fast your soil drains, the area of roof and paving draining to it, and the local design rainfall. Change any one and the size changes — which is why a "standard" soakaway size doesn't exist. A free-draining sandy plot might need a modest pit; the same roof on slow clay can need several times the storage.
Why rules of thumb don't cut it
You'll see "1 m³ per X" rules online, but Building Control won't usually accept them — they ignore your actual ground and rainfall. The recognised method is BRE Digest 365, which balances what flows in during a real design storm against what your soil can soak away, and takes the worst-case storm duration.
Start with a percolation test
The one measurement you need is how fast your soil drains, from a percolation test — three fills in a trial pit. SoakMonkey turns those into the infiltration rate, then sizes the soakaway for your catchment and rainfall and shows the dimensions that pass. It's free to size on screen; the Building Control-ready PDF is £25 +VAT (£30 inc VAT).
Then pick a construction
The same storage can be a big rubble pit or a compact crate soakaway — the construction changes the hole size, not the storage required. SoakMonkey sizes all four so you can see which fits your plot.
Frequently asked questions
What size soakaway do I need?
There is no single answer — it depends on three things: how fast your soil drains (your percolation rate), the area of roof and paving draining to the soakaway, and the local design rainfall. A free-draining sandy site might need a modest pit; a slow clay site can need several times the storage for the same roof. BRE Digest 365 combines those factors to give the right size, and SoakMonkey does that maths for you.
Isn’t there a simple rule of thumb, like 1 m³ per something?
Rules of thumb are unreliable and Building Control won't usually accept them, because they ignore your actual soil and rainfall. Two identical extensions can need very different soakaways depending on the ground. The proper method is BRE 365, which sizes for a real design storm and your measured infiltration rate.
How do I find out how fast my soil drains?
With a percolation (soakage) test: dig a trial pit, fill it with water three times, and time how long the level takes to fall. SoakMonkey turns those three fills into the soil infiltration rate BRE 365 needs, and adopts the lowest (safest) of the three.
Can I work the size out for free?
Yes. Sizing the soakaway on screen — trying different constructions, seeing the dimensions update — is free. You only pay £25 +VAT when you export the Building Control-ready PDF.
Ready? Open the soakaway calculator, or if this is for an extension, start there.