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House purchase drainage surveys

Being a home buyer can be a combination of exciting and stressful and if you are buying a house with a cesspit, septic tank or treatment plant then, for your peace of mind, you would be wise to have the system surveyed. Even the most thorough of homebuyer surveys don't include a survey of private sewer systems and these can be a hugely expensive thing to repair or replace if they have been neglected.

Different homes require different types of survey.

A home is often the most expensive purchase people ever make. If you have a private drainage system, you are known as “the operator” according to the environment agency's new binding rules. By employing ASL to carry out a survey, you will benefit from expert advice on your private sewage system condition.

ASL has the expertise to carry out a specialist survey, focussing on private systems, not to be confused with a CCTV survey of the drains around the house.

If you buy a property with an existing private sewage system without having a survey carried out, you could be inheriting several problems that you will then be responsible for rectifying. Equally, if you sell a property with a cesspit/cesspool, septic tank, or treatment plant, it is your responsibility to disclose full information about it. 

An old septic tank, cesspit or cesspool with a discharge or a clinker bed treatment system discharging into a watercourse was required to be registered with the environment agency by 2020. If you don't know what condition your private sewage system is in, you could face legal penalties for not fully disclosing its condition.

  • The property may have a completely out-of-date sewage system, which will need an expensive upgrade.
  • The property may have the wrong system completely.
  • The system may be discharged into an area which has changed over the years, causing pollution.
  • The system may not meet the new regulations or meet the changes to the binding rules.
  • A house with a cesspit that has an 'unlawful discharge' would need to be capped off.

You can read about the binding rules on the Environment Agency website, and if you have any questions at all, you can call our friendly office team who will help you understand what is expected of you as an 'Operator'. 

What is a cesspit?

A cesspit, also known as a cesspool, is typically around 10,000 litres. A huge tank for storing raw household sewage. It is below ground and watertight. Cesspits need to be emptied every 9 to 13 weeks, depending on the household's usage and size. It is your legal right to have a cesspit to manage household waste, although not recommended. If the property is within reach of the main drain connection, a cesspit would no longer be allowed. To serve a modern family, cesspits are only installed when nothing else is available or possible.

More information on cesspits.

What is a septic tank?

A septic tank also deals with household waste, underground, but it discharges wastewater that has been separated from solids to a land drain. A land drain, also known as a drainage field, requires suitable ground to be effective. If the ground is not suitable for drainage; there is no room for land drains or a mains connection within reach, the only option for disposal of household sewage from the property would be a cesspit. 

More information on septic tanks.

What is a treatment plant?

A treatment plant again deals with household waste in the absence of a main drain connection. However, a domestic treatment plant consists of three different chambers that pass the sewage through different stages of separation before producing clean water to a high enough standard that would discharge to a drainage field or be discharged into rivers, streams or a watercourse.

More information on domestic treatment plants.


Private sewage systems are governed by 'general binding rules' and new laws about the standards that need to be upheld came into force on the 1st January 2020. If you are new to private sewage systems or were not aware of these rules, you can read more about them here on the Gov.uk website.

Cost implications of having to replace or update a private sewage system

Our typical charge to install a 4000-gallon cesspit is over £20,000.00 plus VAT. This does not include sheet piling, de-watering and mucking away, and is, of course, subject to a test hole and a proper detailed estimate.

A survey of a private drainage system is financially vitally important if you are purchasing a house which is not connected to the main sewer.

If you are selling a house with a private sewage system, the vendors can prepare for the sale and may be able to increase the value of the property, or the purchase price by having a survey carried out with a full report of our findings. Both parties will be able to make an informed choice, i.e. it may be possible to revise/reconsider the purchase price or offer and gain knowledge.

It would also be possible to organise and plan when to move in or out, whether to have any work required carried out before moving in or, in the worst-case scenario, walk away from the deal. If the survey reveals the wrong system has been installed, or it’s out of date, or could cost upward of £20,000 to replace it – with enormous disruption, a decision can be made whether or not to go through with the purchase at all.

A prospective vendor may choose to prepare for the sale with a professional, in-depth visual inspection of the private sewage system. Our report provides an expert account, validates the system's condition, and highlights any problems clearly and easily to understand written and verbal reports.

Assets for Vendors and Purchasers.

HomeBuyer Report gives a market valuation of the property and the insurance rebuild costs. It also advises on defects that may affect the value of the property. It does not include an evaluation of the private sewage system. It’s well worth the investment in an independent survey of the drainage and sewer system. Essential for larger or older properties, or if you’re planning major works.

Even if you are stretching your finances to afford the property, it is, essential to get a proper survey to highlight any potential problems. The good news is that even if it does throw up some minor issues with the drainage system, you can often use these to reduce the price – assuming you want to go ahead with the purchase. The advice you receive is impartial and reliable.

David Dalby of the Royal Institute of Chartered Surveyors (RICS) said:

“In difficult economic times, it pays to be prepared. Nobody wants to be left with a home that needs extensive repairs or that they can’t sell on. By carrying out a survey, you’ll be armed with information that puts you in a stronger position to decide whether to proceed with the purchase or negotiate a better deal.”


If a property is being sold, the operator must give the new operator written notice of the small sewage discharge being carried out, and describe the wastewater system and its maintenance. As the operator, you must make sure the sewage system does meet the General binding rules.

Here are some examples of reports that have been sent to clients who have authorised a drainage survey on a private system before and after purchase, including recommendations and costs.

Our survey saved this homebuyer thousands in the wrong solution

Further to your request, we attended the above address. We carried out our investigations and believe there is insufficient room. Based on the boundaries given, there is not enough suitable ground for the septic tank to discharge into a drainage field. The secondary treatment from a septic tank requires light aerated soil and depending on tests, approximately 40 linear meters of trenches, ideally 600mm-1000mm deep. This is subject to a test hole.

Our survey saved this homebuyer £11,000

Therefore treatment of the sewage water is advised. Boreholes could be our only way of discharging. Please budget in the region of £11,000 plus the boreholes.

Our survey saved this homebuyer £6,000

ESTIMATE (boreholes) 

  • To trace area for services and utilities
  • To excavate and carry out a test borehole to a max depth of up to 13m. If the ground conditions at this depth are found to be favourable, we will continue with the following:-
  • To excavate and construct a total of 3 x boreholes (including test borehole) to a max depth of up to 13m. If boreholes are suitable please budget in the region of £6000.
Our survey saved this homebuyer inheriting very expensive problems

A survey carried out because sewage had been forced through to the clean chamber:

We found the covers to the chambers located in the grass/lawn area are also not sealed and could allow surface water to enter and overwhelm the treatment plant and disrupt any treatment, as a result, untreated sewage water, containing food debris etc. pass straight through into the land drains, which eventually over time, will coat the absorbent surfaces until the drainage field ceases to work altogether.

Our survey saved this homebuyer £1,800

PLEASE NOTE: If we do not find adequate soil porosity to a depth of 13m, there will be a one-off charge of £1800.00 plus VAT for the borehole test and we will provide you with a quote for an alternative solution.

Our survey raised the value of this house seller's property.

This house owner was pleased and increased the value of the house and the price:

We have reason to believe that the mains public sewer runs along the bottom of the garden we recommend we contact the local authority and obtain plans confirming and showing the location and depth of the main public sewer. 

This customer was delighted he had not completed the purchase.

This customer was disappointed but delighted because he had not completed the purchase of the property. 

We carried out a search reference test hole to check the ground conditions in your area and found it unfavourable for the existing tank. We have revised our estimate/report accordingly. 

This homeowner was delighted with the result of this report.

There is a possibility that the property could be connected to the main sewer via a specially designed pumping station, utilising the existing drains that lead to the public sewer.

Our survey saved this homebuyer £16,000

This customer was very pleased that he had a house purchase survey

The septic tank is discharging to clay ground covered by 300 mm of topsoil, The sewage water has gone black like oil

We recommended:

A 'state of the art' domestic sewage treatment plant, to treat sewage water to the standards required by current environmental legislation, discharging clean water into the environment, rather than separated but untreated raw liquid sewage. Budget in the region of £16,000 plus VAT.

Our survey saved this homeowner upwards of £20,000

Rainwater in the System upsets the treatment.

On-site, we found the chamber covers to the treatment plant, are not fixed, and therefore allows rainwater into the plant, which will disrupt treatment. During medium to heavy rain, several cubic meters of rainwater can fall in the area, per day. The water will run off the higher ground to the lower, flat ground and into the Treatment Plant, via the covers.

An important component of a treatment plant system is the air compressor. These units pump air into the tank, activating the sludge process, promoting the microbial growth in the wastewater.

On inspection of the compressor housing, located at the bottom of the slope, we found this vulnerable to weather. It could become damp during wet conditions, which would result in a fuse blowing, immediately stopping the treatment process and allow untreated water to discharge without the knowledge of the household.

What is included in your house purchase survey?

We provide you with a full written report of the drains' condition, backed up with a DVD recording of any damage found and a diagram mapping the system layout. We highly recommend that a survey be carried out on the drainage system before the exchange to identify any defects and ensure you know the costs to fix them.

  • We will take the hassle out of arranging the survey for you and contact the estate agents to arrange a suitable appointment.
  • A full detailed report will be provided and emailed to you within 48 hours if the survey is dependent on a sale.
  • The report will be backed up with a schematic diagram and a DVD of any defects found.


Don’t delay we can help you today!

Call 0800 181 684 for all your drainage needs.

Our HQ is based in Guildford, but we cover the whole of Surrey, Parts of Berkshire, Hampshire, West Sussex and South West London.

See the map on our 'areas covered page' for our services reach and use our postcode checker.

Need a drainage survey for a house sale or purchase?

Send us a message below and, we will get back to you ASAP.
Does the property have a private sewage system?
(eg. a cesspit, septic tank or treatment plant)
Are you happy for us to contact you with information in the future?

FREE Quick Look tank inspection with a regular septic tank emptying from ASL Limited

ASL Limited Certifications

Our engineers have a variety of certifications, as follows:-

  • New Roads & Street Works
  • EUSR Water Hygiene
  • CSCS
  • NPORS Excavator & Dumper
  • Confined Space

We are also Safecontractor approved, as well as being part of Which Trusted Traders and Check-A-Trade.