Building living quarters at bottom of garden

not going to get planning, you have a house they'll never give you planning to build another house so you can rent out your own house

The fact he has a house shouldn’t be relevant to planning here. People who have houses develop their property, for the purposes of renting/selling all the time. Either the land is suitable for another dwelling, or it’s not.

Obviously, there are “Local” policies that require a housing need for one-off housing. But that’s not something that applies nationally, and is not found in planning law.

So maybe 18m or less after the house is built at the end of the garden. You’ll need to be careful of overlooking distances.

In general, there should be adequate separation (traditionally about 22 m between 2-storey dwellings) between opposing first floor windows. However, relaxation of this standard will be considered where the careful positioning of opposing windows can prevent overlooking even with shorter back-to-back distances.