Plan around the length of a visit.
An occasional overnight stay and a month-long visit need different amounts of storage, cooking space, laundry access, and seating. Think about the guests you actually expect, including whether two people will keep different schedules.
Make the arrival comfortable.
Consider the walk from parking or the main home, the visibility of the entrance, lighting, and where visitors can set down luggage. Place doors and windows so both households can enjoy their own privacy.
Give the patio a purpose.
Decide whether the outdoor space is for morning coffee, shaded dining, conversation, or access to an existing pool. A clear use helps determine furniture, circulation, shade, and the position of doors. Outdoor features are scoped and priced separately where applicable.
Distinguish a pool house from a dwelling.
A changing room, recreation space, and complete guest dwelling can involve different features and review requirements. Define the intended building use and plumbing, kitchen, and sleeping needs before treating them as the same project.