What size septic tank do I need? (Approved Document H)
The size Building Control actually checks — and why three different formulas are doing the rounds.
There are three septic tank formulas circulating in Britain, and they give different answers. Only one is the one Building Control opens when it signs off your installation: paragraph 1.18 of Approved Document H.
It reads, word for word: “Septic tanks should have a capacity below the level of the inlet of at least 2,700 litres (2.7m³) for up to 4 users. The size should be increased by 180 litres for each additional user.” That is the whole rule. You can read it yourself — the official PDF is on gov.uk, paragraph 1.18.
The other two formulas you will meet are not inventions, they are just not the regulation. British Water's older “Flows and Loads” method multiplies people by 150 litres and adds 2,000. Several installers publish “180P + 2,000”. At four people the three answers differ by a couple of hundred litres — enough to argue about, not enough to matter. At eight people the gap grows.
Two things the number does not tell you. First, the capacity is measured below the level of the inlet — not the tank's total volume, which is what a seller's brochure shows. Second, a bigger tank does not rescue a bad drainage field: if the ground cannot take the effluent, litres in the tank change nothing.
And the tank itself has a product standard, which is a different thing again: paragraph 1.19 says factory-made tanks “should meet the requirements of BS EN 12566-1”. BS 6297 is neither of these — it is the 1983 code of practice cited in paragraph 0.4 as specialist design guidance.
FAQ
What size septic tank for a 4-bedroom house?
The rule counts people, not bedrooms. Approved Document H §1.18 sets a minimum of 2,700 litres for up to 4 users, plus 180 litres per extra user. A 4-bedroom house that sleeps 6 needs 2,700 + 180 × 2 = 3,060 litres as the regulatory floor.
Why do I keep seeing different formulas?
Because three are in use: Approved Document H (2,700 + 180 × (P − 4)), British Water Flows and Loads (P × 150 + 2,000) and a commercial variant (180P + 2,000). Building Control references Approved Document H — that is the one that decides sign-off.
Is 2,700 litres the total tank size?
No. Paragraph 1.18 measures capacity “below the level of the inlet”. A brochure quoting total volume is measuring something else, so compare like with like before you buy.
Does a bigger tank fix a soggy drainage field?
No. The tank settles solids; the ground does the treatment. If your percolation value is outside 12–100 (Approved Document H §1.38), no tank size makes a drainage field work — you need a different filière.