What you describe, Freddy, is likely to be classed as a "granny annexe."
An extension to the main house (it doesn't have to be "attached")
Generally regarded as "Permitted Development"
It's easy to check with your local planning dept. though. It would be worth it, as there are exceptions to PD.
For instance... Listed Building, Conservation Area, Restrictive Covenants etc.
Planning Depts vary a lot even though they should all be using the same hymn sheet ;o(
Some might be reluctant to stick their necks out. In which case they'll ask for an application for a "Certificate of Lawfulness"
There is a fee of course, but the procedure is quite straightforward.
A C of L is just an official acknowledgement that you don't need Planning Permission. Nothing to be afraid of.
More important is compliance with Building Regulations. What you describe is "Notifiable".
For small works such as this, a "Building Notice" is by far the easiest and cheapest way to comply with the Regs.
Go for it ;o)