Building over or within 3m of a public sewer normally requires a build-over agreement with your water and sewerage company, and that agreement usually requires a CCTV survey of the run first, plus protective measures such as a lintel over the pipe.
Skipping this at build time is one of the most expensive shortcuts in domestic construction. It surfaces when the property is sold, when the buyer's solicitor asks for the agreement and it does not exist.
If you are unsure whether a drain is public or private, the water company's sewer records will show it. We can survey the run and provide the report the application needs.