In town, waste water goes to the sewer and nobody thinks about it again. In the countryside, in a hamlet or at the end of a lane with no sewer, the plot itself has to treat it. This is off-mains drainage: a septic tank with a drainage field, a small sewage treatment plant, or in some cases a cesspool that simply stores everything until it's emptied.
This guide doesn't replace the percolation test, the installer or the regulator, and the rules vary a lot between England, Wales, Scotland, Northern Ireland and Ireland. It's there to help you understand what you're being offered and to prepare a clean plan, before and after the work.
The principle: three stages
Every system follows the same pattern, even if the equipment differs.
- Collection. Water from the WCs and from the kitchen, bathroom and washing machine leaves the house together towards the system. Rainwater has no business there: it would overload it. It goes to a rainwater tank, a soakaway or a ditch.
- Primary treatment. In a septic tank, solids settle to the bottom, fats float to the top, and a first digestion happens without oxygen. The liquid that leaves is clearer, but not clean.
- Secondary treatment, then discharge. The liquid is cleaned by the soil (a drainage field), by a sand filter or a filter medium, or inside a treatment plant. The treated water then soaks into the ground, or goes to a ditch or a stream where that's allowed.
The main types of system
| System | Principle | Footprint | Electricity | Maintenance |
|---|---|---|---|---|
| Septic tank + drainage field | The natural soil filters and treats | Large: trenches over tens of m² | No | Emptying the tank |
| Septic tank + sand filter or mound | Imported sand does the treatment | Medium | No, unless a pump is needed | Emptying, checking the filter |
| Compact filter | A medium (coconut fibre, mineral wool, zeolite…) in a tank | Small | Depends on the model | Emptying, replacing the medium |
| Small sewage treatment plant | Bacteria with air pumped in, inside a tank | Small | Yes, all the time | More frequent emptying, servicing the equipment |
| Reed bed | Planted beds of reeds | Medium to large | Depends on the model | Cutting back, little emptying |
The right system depends on the soil, the space, the slope, where the treated water can go, the number of occupants and what's permitted where you live.
What decides where it goes
The installer or the drainage engineer makes the final call, but your plan helps a lot if it already shows:
- The foul drain leaving the house: under which wall, at what depth. The further or higher the tank, the more fall you need, or a pumping station.
- The slope of the ground. Ideally a system runs downhill: house, tank, treatment, then outfall.
- The soil. Clay, sand, rock, a high water table: the percolation test tells you whether the ground can take the water. A corner of the garden that's always waterlogged in winter is a bad spot for a drainage field.
- The outfall: where the treated water can go. Into the ground, a ditch, a stream: each has its own rules.
- Free space. Not under a patio, a driveway or a future shed, and not where you dream of putting a pool.
The distances to check for your plot
Minimum distances are set by the rules and guidance of your country, and they change with the system and the soil. In England, Approved Document H, the official guidance on Part H of the Building Regulations, sets them out for septic tanks, treatment plants and drainage fields. The table lists what to measure on your plan, not the values to meet: ask the installer or building control for the figures, and add them to your plan.
| Distance to measure | Why it matters |
|---|---|
| System ↔ house and foundations | Damp, stability of the foundations, smells |
| System ↔ plot boundaries | Protecting the neighbour, access for maintenance |
| System ↔ well, spring, borehole | Not contaminating water that someone drinks or uses |
| System ↔ rainwater tank | Keeping reused water and waste water clearly apart |
| Drainage field ↔ trees and hedges | Roots clog and lift the pipes |
| Drainage field ↔ watercourse, ditch | Treated water mustn't run straight into it |
| Drainage field ↔ pool, veg patch, paved areas | The ground above has to stay permeable and lightly used |
| Tank ↔ access for the emptying tanker | The suction hose has a limited length |
One example of why a well belongs on your plan: in England, according to gov.uk at the time of writing, the general binding rules for small discharges to ground can't be used in a groundwater Source Protection Zone 1, which includes any area within 50 m of a private water supply for human consumption. In that case, you need a permit from the Environment Agency.
Chambers, lids and tanker access
Once the grass has grown back, only the lids and the inspection chambers stay visible. They're what make maintenance possible.
- An inspection chamber where the drain leaves the house, to clear a blockage without opening the tank.
- The lids of the tank and, depending on the system, of the treatment unit, at ground level, never buried under a patio or soil.
- A distribution chamber at the head of a drainage field, to check the flow is spread evenly, and often a chamber at the far end.
- A sampling point at the outlet, where checks on the treated water are expected.
A septic tank or treatment plant is emptied (desludged) regularly, by a tanker that sucks out the sludge. In England, the gov.uk guidance at the time of writing says to do it at least once a year, or as the manufacturer's instructions say, using a registered waste carrier. The tanker has to get close: ask the emptying firm the maximum distance between the tanker and the lid, and check on the plan that the route stays usable, with no gate that's too narrow, no soggy lawn to cross and no future carport in the way.
What not to do on top
A drainage field or a buried filter needs air and loose soil. A lawn on top is ideal. Avoid:
- driving or parking on it;
- trees and large shrubs, whose roots block the pipes;
- sealed surfaces: patio, concrete path, shed on a slab;
- the vegetable patch, to be safe;
- tipping soil on top, which changes the depth of the pipes.
Draw the area on your plan, even if you can't see it: it's what protects the system the day someone wants to plant a hedge.
An existing system: find it and record it
Just bought the house, or nobody remembers exactly where the tank is? A few clues:
- lids in the lawn, sometimes hidden under a few centimetres of soil;
- the foul drain leaving the house, in a cellar or at the foot of a wall;
- a strip of greener grass in summer, which often gives away the drainage field;
- the sale documents: the drainage answers in the property information form, the seller's maintenance records, an old building control completion certificate.
In England, the seller has to tell the new owner in writing that there's a sewage discharge, with details of the system and its maintenance. Record each structure as soon as you find it, with its source: “lid found by probing”, “route assumed from the grass”. The guide mapping underground pipes and cables explains how to note the depth of a pipe and how sure you are.
The paperwork, in broad terms
- In England, the Environment Agency's general binding rules cover small discharges, at the time of writing up to 2 m³ a day to ground, or 5 m³ a day to surface water from a treatment plant. A septic tank must not discharge straight into a ditch or stream: it has to be replaced or upgraded. If a public foul sewer is within 30 m of your boundary, you can't rely on these rules: you connect where that's reasonable, or apply for a permit. Above the volume limits, you need a permit too. A new installation also needs building regulations approval, and building control will usually want a percolation test.
- In Wales, Scotland and Northern Ireland, the regulators are Natural Resources Wales, SEPA and DAERA, each with its own rules and registration.
- In Ireland, every domestic waste water treatment system has to be registered with the local authority, and the councils carry out inspections under the EPA's national inspection plan.
- Everywhere, ask first whether a sewer is planned for your road: a new system two years before it arrives is an expensive mistake.
Draw it in Architektor
Switch to the underground level. The house, the boundaries, the drive and the trees stay visible, faded: that's what makes the distances readable.
- The tank or the treatment plant: Tank / manhole tool (K key), type “Septic tank / treatment plant”. Set its shape, dimensions, depth and volume. If the tank and the treatment unit are two separate tanks, place two elements and name them.
- The chambers: same tool, type “Manhole / inspection chamber”, at each change of direction and at each tank inlet and outlet.
- The pipes: Pipe tool (U key), service “Sewer / wastewater”, from the house to the tank, then on to the treatment. Enter the diameter and the depth at the chambers.
- The drainage field: Tank / manhole tool, type “Soakaway”, with the real area of the field. For trenches, you can also draw each pipe with the Pipe tool.
- The distances: add permanent dimension lines (D key) between the system and the house, the boundaries, the well and the rainwater tank. They're what the installer and building control look at first.
- The access: on the garden level, draw the tanker's route with the Area tool, type “Driveway / parking” or “Path”.
For a project, set the new elements to “To build” (red) and an old tank to remove to “To demolish” (yellow). Each element's Notes field keeps the useful details: make and model, date installed, date last emptied. These notes are never shown in a share.
Share the plan and keep it up to date
Before the work, send the plan to the installer or the drainage engineer with the “Underground utilities” link: they see the services without the house details, measure distances and pin comments on the plan. Building control and the regulator usually ask for a site plan: the PDF to scale exported from Architektor is a good base, which the installer can complete with what's required.
After the work, update the plan with what was really installed, backed by photos of the open trenches, then save a named version, for example “Treatment plant installed, June”. The day you sell or build a shed or a carport, this plan saves you probing the whole lawn. And if the plot boundaries are unclear, the guide title plans, aerial photos and measuring your land helps you place the system correctly in relation to the neighbour.