Septic tank cost guide: Installation, replacement and repair costs

It’s easy to take for granted reliable plumbing in towns and cities, but in the countryside, it’s not always an option. Where mains drainage isn’t available, a septic tank is an ideal way to remove sewage and wastewater from the home.

Best of all, a well-designed system connected to a drainage field can last for decades and even be cheaper in the long run than paying to be connected to the public sewers.

What’s the average cost to install a septic tank?

The cost to install a septic tank ranges from £4,200 – £6,000. This includes excavation, the tank itself and installation.

How much do septic tanks cost?

The cost of your septic tank will be around £1,200 for a small septic tank – £1,800 for a large septic tank.

Septic tank installation labour costs and timings

Labour costs for installation are generally between £1,000 - £1,400, with a further cost of approximately £2,700 - £3,905 for excavation needed for a new installation.

Above-ground systems are very simple to install and generally take a day. They also have lower costs for soil disposal and excavation than below-ground systems.

Below-ground systems and drain fields can take a week or two to install. But this will depend on the size of the tank, the soil quality, and the availability of diggers and excavators.

Additional time will be needed to secure the necessary permits as well.

Health and safety are key considerations with these installations. Due to the deep excavations, earthwork, the considerable size of the tank, and the need to ensure safe access, we recommend that the installation be done by a trained professional.

Septic tank installation hourly rates

Most septic tank installation companies will quote on a ‘per project’ rate for the septic installation labour costs, which typically range from £1,000 - £1,400.

Calculating your septic tank installation cost

If you’re looking to calculate the cost of septic tank installation, you’ll need to decide on the size of the tank you’ll need. You’ll also need to think about the location of where you’ll be installing the septic tank, and what type of material you’ll opt for when it comes to the tank itself.

You can get a good idea of the budget you need to install a septic tank by considering the following:

• The cost of your septic tank: £1,200 (small tank) – £1,800 (large tank)
• Labour cost to install the tank: between £1,000 - £1,400

Here’s what to budget, depending on the size of your septic tank:

• Install small tank: £4,200 
• Install a medium septic tank: £5,000
• Install a large septic tank: £6,000
• Replace a septic tank: around £4,000

If in doubt, speak to your local septic tank specialists to get a breakdown of costs – and ask them to provide options, so you can see how different factors affect the total cost of the installation.

Septic tank replacement costs

To replace your septic tank with a new one, you can expect to pay around £4,000. 

There are many reasons to look into replacing your septic tank:

• The septic tank is broken
• Another house is added to the same private drainage system
• You move into a new home with a larger family than lived there previously
• Your family grows and the current system isn’t big enough anymore

If you can use the existing drainage field for septic tank replacement, it will cost slightly less than installing a new system. You may even be able to sell the old system to recoup some of the costs.

However, if the old septic tank has broken or contaminated the ground around it then costs will be as much as or even more than a new installation.

If the tank has not been pumped enough, the old drainage field can become clogged. In this case, the bacteria that’s built up takes a long time to die out, and you’ll have to pay the cost of an entirely new tank.

Contaminated soil must also be removed. This will cost around £40/m³.

Septic tank repair costs

Good quality septic tanks are designed to last a long time, but they’re not unbreakable.

Here are some common faults and how much they’ll cost to repair:

• Replace distribution box: £750
• Replace the baffle: £500
• Replace the septic tank pump: £750
• Repair a crack in your septic tank: £500
• Replace septic tank lid: £300
• Clear blocked pipes with septic tank jetting: £300

Septic tank repairs cost considerably less than replacing the system.

Regular inspections can help reduce the risk of these issues occurring and aid with insurance claims should the septic tank be unrepairable.

Septic tank maintenance jobs and their cost

There are a few tasks to keeping your septic tank well maintained. These include:

• Regularly emptying the septic tank: £120 - £350
• Sludge judge tool to determine when your tank needs cleaning: £230
• Septic tank jetting to prevent blockages: £300
• Septic tank chemical treatment: £200

Need a septic tank installation quote?

To find a local, reputable, tradesperson in your area to install a new septic system, use our free search feature.

Key takeaways for septic tank installation

• Buying the largest tank you can afford and have space for will reduce the amount you spend later on
• Concrete septic tanks are cheaper than other alternatives but may not last as long as polyethylene ones
• Above-ground systems are cheaper initially and easier to install but can result in considerable emptying costs
• Below-ground systems are more standard and may not need to be emptied for years with lower drainage costs overall
• Be sure to speak with your local authority to get planning permission and follow all septic tank installation regulations

Frequently asked questions about septic tanks

Should I repair or replace my septic tank?

This will depend on the issue with your septic tank, and how much it will cost to repair vs replace your tank.

However, it’s almost always cheaper to repair your septic tank rather than replace it.

Common septic tank repair costs range from around £300 to replace the lid to around £750 to replace the distribution box or septic tank pump.

Meanwhile, a full replacement of your septic tank will cost around £4,000.

How does a septic tank work?

A septic tank stores wastewater from your home and allows it to settle as effluent, leaving the solid contaminated waste (sludge) at the bottom of the tank, scum at the top of the tank and liquid septic water in the middle.

With underground systems, the liquid septic water is filtered and released into a drain field through an outlet pipe where it is treated by microorganisms. The remaining sludge and scum in the tank is left to decompose. Should this build up too much, then the tank will need to be emptied.

What size septic tank do I need?

Septic tanks collect waste not only from sinks, baths and toilets but also from dishwashers and washing machines. This means a considerable amount of water can be collected every single day.

Making sure you get the right size is key, as smaller tanks quickly end up costing more to empty in the long run if you haven’t picked a big enough size.

To calculate the size you need, BS6297 and British Water Flows and Loads recommend the following formula:

Size needed = 150 litres x maximum number of people in the property + 2,000 litres

British Water Flows and Loads estimates each person will use 150 litres per day, but you may want to do your own calculation. Children are treated as needing the same amount as adults. 2,000 litres is the minimum amount of litres needed to comply with building regulations.

By spending more upfront and choosing the largest system you can afford and have space for, you will save more in the long run.

Smaller septic tanks cost around £1,200 to purchase, whilst larger ones can cost around £1,800.

What types of septic tanks are there to choose from?

There are two main considerations in terms of the type of septic tank: what design is best for your garden and what material you want the tank to be made of.

• Design: Gravity systems that use the existing water flow from the pipes are best if you’re installing the tank in an area with rich soil or dirt, while pressure systems are ideal for areas with gravel or coarse soil
• Material: The most common materials for septic tanks are concrete, polyethylene and fibreglass which vary in strength and how long they will last – for instance, polyethylene is very long-lasting but more expensive than a standard concrete system

How do I choose a location for my new septic tank?

You can choose to position a septic tank below or above ground, with the following benefits:

• Above ground: Cheaper and a great option for small systems if you don’t mind them being visible or taking up space. An ideal solution for temporary structures too
• Below ground: More expensive for installation but take up less space and allow you to use larger tanks, saving you money in the long run. Essential for larger families

Are there septic tank installation regulations to follow?

Yes there are. The rules on septic tank installations were updated in 2023, in the form of the General Binding Rules. These state you cannot empty a septic tank into a waterway.

In addition to these rules, you must check with your local authority whether you can install a septic tank. The UK Government has an excellent guide on the General Binding Rules you need to follow and whether you will require a permit.

You must also seek planning permission and building regulations approval. This can be secured through your local authority, who will also be able to confirm if you are in a protected zone.

Do I need a septic tank survey?

For peace of mind when buying or selling a home, septic tank surveys are generally requested to ensure everything is okay with the installation.

You should consider that a poor-quality tank or installation may cost you down the line when this survey is done. Buying a correctly sized, good quality tank that’s properly installed will save money in the long run, every time.

It is the property seller’s obligation to tell the buyer about the septic tank, including:

• A description of the treatment system and drainage system
• The location of the main parts of the treatment system, drainage system and discharge point
• Details of any changes made to the treatment system and drainage system
• Details of how the treatment system and drainage system should be maintained and the maintenance manual
• Maintenance records

For any questions that haven’t been answered, you’re best off chatting to one of your local, Checkatrade-approved septic tank installers.