If a property you wish to buy still has a non-compliant waste management system, ask the seller whether they intend to replace or upgrade it—agree to this as a condition of sale. Alternatively, consider lowering your offer to make up for the future costs of improving it yourself.

If you are selling the property, it is your responsibility to install a sewage treatment system compliant with the general binding rules. Being non-compliant will not only detract potential buyers but you may also be subject to enforcement action by the Environment Agency.

You must also provide the following information to the buyer:

  • A description of the treatment plant and drainage system

  • The location of the main parts of the treatment plant, drainage system and discharge point

  • Details of any changes made to the treatment plant and drainage system

  • Details of how the treatment plant should be maintained, and the maintenance manual if you have one

  • Maintenance records if you have them

Source: https://www.propertymark.co.uk/professional-standards/consumer-guides/buying-selling-houses/septic-tanks.html