Cost of Installing a Septic Tank

All you need to know about the factors that affect septic tank installation costs including costs of materials, labour and time frames.

Septic Tank

What the job entails

Urban dwellers usually take sewer systems for granted, but if you live in a rural area you will likely already know all about septic tanks. If there is no mains sewage then homes need a septic tank to provide a place for waste water to drain into. Septic tanks need emptying at least once a year and there are five main factors you need to consider in terms of cost which can affect the price for the installation or the replacement of a septic tank system.

The first and most obvious cost factor to consider is the size of the septic tank. This will have a direct effect on the cost but it is a false economy to install a septic tank which is too small, as this will just need emptying more often and will end up costing as much, if not more, than a larger septic tank. You need to remember that septic tanks are not just used for sewage, they also take "grey water" from the washing machines, sinks, showers and dishwashers too. In fact, all the waste-water from your home will be draining into a septic tank so you need to ensure the tank has adequate capacity. If anything, you should err on the side of slightly too large, as the extra installation costs would usually be cheaper than more frequent emptying over the long term. A medium septic tank would typically cost around £700-£800, whereas a large tank would be around £1,200.

Whether the tank is above or below ground also has a large impact on the price. Underground septic tanks have additional costs in terms of excavation so above-ground tanks are generally cheaper and the best solution if you only need a small septic tank system. Having an underground tank fitted is not cheap due to the installation costs, but on small sites you may have no other option if surface space is at a premium. In addition, the larger sizes of septic tank tend to be designed for underground usage. When choosing you should always contact the local authority before making a decision, as they may have restrictions in place on the type of septic tank system you can use.

Drainage systems are required for below-ground tanks which can also add to initial costs. But the benefit of a larger underground tank can often be worth the additional initial expense, a the soak-away can allow some of the waste-water to be filtered out slowly into the surrounding ground so the tank is able does not need to be emptied as often, saving you money in the long run. However, not all sites will be suitable for a soak away, the ground needs to be checked to ensure it will be able to absorb some of the excess water. Getting this checked over and having any required pipe and drainage works installed will push up the project’s price.

The material used to construct the septic tank is also a cost factor, as you would expect, the better quality and longer lasting materials are more expensive. But it really is a case of getting what you pay for, so if you can afford the extra initial outlay, it’s worth paying a premium as the tank will last much longer. Cheaper tanks are made of concrete, whereas the premium models are made of a high-density polyethylene material which will last much longer so is more economical over the long term.

The other major cost factor is the labour required for the septic tank installation. Typical installation labour costs are between £1,200 and £1,800 for a straightforward job. This can be reduced if you can do part or all of the excavation yourself by hiring a digger, as it’s relatively simple to dig a hole.

Ready to get a quote from local tradesmen?

Post your job in minutes and get quotes from local and reliable trades. It's FREE. No obligations.

Get a Quote

Installation Cost of a Septic Tank

The average material cost for a medium septic tank would typically cost around £800, whereas a large tank would be around £1,200.

The average septic tank specialist will usually charge around £150 to £250 per day, although labourers will often work in teams and labour costs will be factored into the overall price. A typical septic tank installation is expected to take up to 4-7 days to complete depending on the size of the septic tank and area of excavation.

Below are some estimated costs of hiring a specialist to install an underground septic tank:

Septic Tank Size Avg. Cost Duration
Small £1500 2-4 days
Medium £2000 3-5 days
Large £3000 5-7 days

For above ground septic tanks it shouldn't take more than a day for installation so expect around £200 in labour costs on top of the cost for the septic tank.

Cost Breakdown

Individual costs of installing an undergroud, medium-sized septic tank - Total Cost: £2000






Waste Removal


A septic tank is a multi-chamber system which retains sewage from a property for sufficient time to allow the solids to be naturally broken down. The remaining effluent can then drain from the tank through an outlet pipe. They are usually installed where there is no mains drainage and are buried in the ground.
They are basically just a way of slowing up the sewage to allow sufficient time for the naturally break up of the contamination by enzymes. The septic tank is basically a storage point for sewage and after the solids have settled the liquid effluent on top of the sludge has a relatively low level of contamination which can safely drained into a soak-away and then into the ground. The sludge itself is then broken down over time by naturally-occurring microbes.
The main difference is that septic tanks have no power demand and no moving parts, so they are cheaper to install and run, but the final effluent from a septic tank will always require further treatment in a soak-away. Whereas waste-water treatment plants usually have an electricity supply which drives a small pump to aerate the waste-water and break it down so that no soak-away is required.
Although most of the sludge in the bottom of the septic tank is broken down by bacteria, it is likely that some sludge at the bottom of the tank will remain and this will build up over time so needs to be removed. When you hire a contractor to empty the tank they will take the sludge but put the watery effluent back in as this contains the good microbes necessary for your septic tank to work properly. It’s really not emptying your septic tank, but simply removing the sludge.
The addition of bacteria into the septic tank augments the existing biological population and ensures that the tank has all the right strains of bacteria to work efficiently. In a perfect environment this is not necessary as the biomass will be self-sustaining, but some strains of bacteria find it harder to thrive in a septic tank than others, so regular bio-augmentation guarantees your tank has sufficient bacteria of the right kind in it.
Only minimal maintenance in the form of a once yearly pressure wash is normally required. However, any oil stains or spills should be cleaned professionally asap to prevent permanent stains. Weed growth on block paving does not come from below the block but from surface seeding caused by airborne seeds becoming lodged into any small gaps in the paving. This can be minimized by spraying with a weed killer twice a year and keeping the surface well sealed.