Unauthorised development - converted garage

[Temor]

Im hoping you can advise me on this query. A friend of mine bought a house a few years ago, the garage was already converted into a bedroom. He recently renovated the house and extended the (garage) bedroom to include a kitchen and living area. He got planning permission for this. There is only one main entrance door into house. Inside main door is a door to main house and a door to the bedroom/kitchen/living area'. He now got a letter from council stating possible unauthorised development and they need to inspect the building. Anyone had a similar experience or point of view on this?
Thanks

[pico]

Is the room / kitchen / living area being let out?
Was there a condition in the Planning Permission regarding occupying the house as a single dwelling unit?