The least risky first use case is family accommodation: an adult child saving for a deposit, a parent who needs nearby support, or a household that needs independent space without a full move.
For family use, compare layout, bathroom access, heating, sound separation, step-free access, fire safety, privacy, and how the unit connects to the main house services. The cheapest layout is rarely the right layout if it fails daily living needs.
Ask suppliers to separate the unit specification from site works and compliance tasks. If the unit may later be rented, keep that future use in the questions, but do not assume a family annex automatically becomes a compliant rental unit.