Under common law, as a professional person in this line of business, you would be expected to know, or to be in a position to advise specifics about the work you do.
As such, in this instance you would be expected to check if PP was required (and building regs, and anything else related to building it, but not covenants or easements) and advise the customer accordingly, or at the least advise the customer that a check needs to be done to confirm if PP or other permissions are required or not. And you should only start once this has been done.
Whilst it is the homeowner who would be ultimately responsible for the consequences of not getting any permissions, they would have a valid claim against the professional person for not giving advice or information which they could be expected to have known. So they would seek to claim any losses back from you.
If you are just the neighbour doing a bit of DIY to help them out, then the above would not apply