How the SepticSpec calculation works
For an England domestic planning estimate, SepticSpec keeps four different design questions separate: the design population, the septic tank capacity, the daily discharge volume, and the drainage-field area. That prevents one familiar figure being used for the wrong regulatory decision.
1. Design population (P)
For a single house, British Water Flows & Loads uses a minimum 5P design load up to and including three bedrooms, then adds one person for each additional bedroom. The Environment Agency also uses bedroom-based population when estimating domestic sewage discharge volume.
2. Septic tank capacity
Approved Document H, paragraph 1.18, says septic tanks should have at least 2,700 litres below inlet level for up to four users, increased by 180 litres for each additional user. A 5P design load therefore gives 2,880 litres; 6P gives 3,060 litres.
3. Daily flow
The Environment Agency domestic sewage risk-assessment method uses 150 litres per person per day. SepticSpec shows P × 150 beside the tank result because discharge-volume thresholds and tank capacity are not the same question.
4. Drainage field
Approved Document H sizes the drainage-field floor area using Aₜ = P × Vp × 0.25. Vp is measured in seconds per millimetre using the prescribed site test. The familiar 12 to 100 sec/mm band is only one suitability test; groundwater and site constraints still matter.
Key siting checks from Approved Document H
- Septic tank: at least 7m from habitable parts of buildings and preferably downslope.
- Drainage field: at least 10m from a watercourse or permeable drain.
- Drainage field: at least 50m from a groundwater abstraction used for water supply.
- Drainage field: at least 15m from a building.
- No access roads, driveways or paved areas within the disposal area.
- Parallel drainage trenches should retain 2m of undisturbed ground between them.
What about direct watercourse discharges?
In England, current Environment Agency guidance states that a septic tank must not discharge directly to surface water. An existing direct septic discharge needs a compliant alternative route, such as a suitable treatment plant discharge where the rules can be met, a drainage field to ground, or connection to foul sewer.
Frequently asked questions
Is the smallest tank always 2,700 litres?
Approved Document H uses 2,700 litres for up to four users. A normal single house sized at 5P or more produces a higher figure. Select a manufactured system whose applicable rated capacity is not below the planning requirement and confirm the manufacturer's design basis.
Can I use the number of people currently living in the house?
For design, use the maximum population load the property is intended to serve. Temporary under-occupation should not normally be used to justify a smaller permanent wastewater system.
Can I use a rainwater soakaway test?
No. A foul drainage field uses the Approved Document H / BS 6297 percolation method. A surface-water soakaway is a different drainage problem and is commonly assessed using BRE 365 methods.
Does a Vp between 12 and 100 mean my site passes?
No. It removes one obvious percolation-rate blocker only. The trial-hole groundwater assessment plus siting, environmental and regulatory constraints still apply.