Planning & Development
You may not have been flooded but this section affects everyone ...
If you intend to submit a planning application to extend your home, you must manage your surface water on site, for guidance (and to avoid disappointment) refer to the relevant Local plan (LP) and include SuDS.
Additional SuDS info.
Firstly, refer to section 19 of the National Planning Policy Framework (NPPF).
Next (if in the London Borough of Croydon), refer to section 5 of the London LP.
Then the Local Plans, followed by the Neighbour Plan (NP if applicable).
Incidentally, Croydon is the 4th worst area in England and wales for surface water flooding, hence the 'watertight' policy.
When converting your lawn to drive...
nb 'Permeable' includes block paving, Tarmac or concrete are NOT, failure to seek planning permission for NEW drive, could result in a planning enforcement issue.
If you have a drive with a gulley or Aco drain at the boundary, it is your responsibility to clean it, so surface water is not permitted to discharge onto the highway.
Incidentally a gutter routed to discharge over a pavement is illegal.
Refer also to your responsibilities.