It can usually be found in the basement, garage, or crawl space. You will want to look for a pipe that is about four inches in diameter with a screw cap on the top with a notch or a square knob at the top. Note that some houses have an indoor sewer line cleanout point, while others might be located outdoors.May 29, 2020,
Assuming a detached property in London has no shared drains, then the homeowner is responsible for the drains up to the property boundary, and Thames Water is responsible for all blockages or repairs off your property.May 2, 2020
One way to find out if your property has surface water drainage is checking your property's Title Deeds (you can do this through Gov), or looking at your original Planning Application.May 30, 2019
How to get sewer maps. You can visit your local authority's offices to inspect their public sewer map, or you can order an asset location search from the water company in question (many of them offer this online through Digdat).
For indoor main drains, you will likely find the cleanout in a bathroom or utility area. When dealing with a bathroom location, check the floor near the toilet. In this scenario, it might be a pipe protruding from the floor or it might be flush mounted into the floor. These main drains tend to have a threaded plug.