We help you choose a solution and build the external sewer for new builds in Riga and the Riga region: a mains sewer connection, a septic tank, a biological wastewater treatment plant (BIO plant) or a holding tank. External sewer installation costs from €25/m, a septic tank with installation from €2,200.
Step 1: is there a mains sewer in the street
First, check with the local water utility whether there is a mains sewer network along your plot and on what technical conditions you can connect. If there is a network, a connection is usually the simplest solution for the whole life of the house, and in Riga municipal co-financing is available for it. See the page connecting to the mains sewer.
If a network is planned but not yet built, the building code allows a sealed holding tank as a solution until the connection is made. If there is no network and there will not be one in the coming years, you need a decentralised system.
Step 2: no network? Choose according to the plot
| Plot | Most suitable system |
|---|---|
| Sandy soil, deep groundwater, enough space | Septic tank with a drain field (plastic septic tank installation) |
| Clay soil | Biological treatment plant or a septic tank with a sand-gravel filter and an outlet (septic tank in clay soil) |
| High groundwater | Anchored tank, raised field or biological treatment plant (septic tank for high groundwater) |
| Small plot or nowhere to discharge the treated water | Biological treatment plant with an outlet to a ditch, or a holding tank (holding tank installation) |
For a new build, the geotechnical survey report already shows the soil layers and groundwater level, so use it when choosing the sewage system too. That is cheaper than rebuilding a system that does not work a year later.
Step 3: documents and registration
The external sewer can be included in the house's building design or documented separately. For a small wastewater treatment structure of up to 5 m³ per day, an explanatory memorandum is usually enough; the exact scope is set by the municipal building authority. Registration deadlines differ between municipalities: in Riga, the system is registered within a month of construction or commissioning, while in Mārupe Municipality it is registered before the building is commissioned. For a septic tank, the registration must include the infiltration system layout drawing and the installation and operation documentation. So keep the layout drawing from day one. For how to register, see registering a decentralised sewage system.
A small wastewater treatment structure of up to 5 m³/d is a Group I engineering structure requiring an explanatory memorandum (LV portāls, e-consultation 06.12.2024; Cabinet Regulation No. 253, point 94.2). LBN 223-15, point 152: holding tanks where a mains sewer is planned but not installed. Riga City Council Binding Regulations No. 66, point 4.2; Mārupe Municipality Binding Regulations No. 13/2023, point 9.2 (documents for septic tank registration) and point 10 (registration before commissioning). Sources: likumi.lv, lvportals.lv, checked on 30 October 2026.
Order of work with the builders
- Before the foundations Decide where and at what depth the sewer pipe leaves the house, and lay a sleeve through the foundations or under the floor slab. Drilling through the foundations later is expensive and risky.
- Pipe depth A gravity pipe must be protected from frost. In Riga, in clayey soil, this means about 0.9 m to the top of the pipe; shallower pipes are insulated.
- Electricity A biological treatment plant or a pump needs a cable from the house. Lay a cable duct together with the sewer, before landscaping.
- Location of the tank and field Not under a future driveway or parking space, with access for the vacuum tanker, away from the well and the house foundations. Fence off the drain field area during construction so that machinery does not compact it.
- Installing the tank After the heavy work, once cranes and concrete mixers no longer drive over the area, but before paving and lawn. During construction, workers can use a portable toilet. To read about pumping it, see the page portable toilet pumping.
- Rainwater separately Never connect roof gutters or land drainage to the septic tank, because they overfill the tank and wash sludge out onto the field. They need their own system.
LBN 223-15, point 67: for gravity pipelines up to 500 mm, the depth to the outer surface is 0.3 m less than the once-in-100-years frost depth, but not less than 0.7 m. LBN 003-19, Table 15: in Riga, the once-in-100-years frost depth for clayey soil is 120 cm. Sources: likumi.lv, checked on 30 October 2026.
How much a sewage system for a new build costs
External sewer pipe installation costs from €25/m, so the length from the house to the tank or network has a big effect on the total price. A septic tank including the tank and installation costs from €2,200. A biological treatment plant for 4–6 people costs from €1,750, installation work from €800. If the house is lower than the network or tank and the wastewater has to be lifted, a sewage pump station costs from €1,650. The price is affected by the soil, groundwater, digging depth and whether the work can be done together with other groundworks.