Planning rules in New South Wales

Yes. A secondary dwelling in NSW can be rented to anyone, not just family, provided it was approved as a separate dwelling.

There is no single figure. Most councils map a minimum lot size for dual occupancy between 450 m² and 600 m², and in mapped low and mid-rise housing areas a 450 m² lot at least 12 m wide cannot be refused on lot size grounds. Some councils, including Hornsby and Ku-ring-gai, require 700 m² or more. Check the minimum lot size mapped to your address before you buy or design.

There is no statewide figure. Your council's Local Environmental Plan maps a minimum lot size for your specific parcel - commonly 450-700 m² in standard residential zones, but as low as around 200 m² in some areas and several hectares in others. In practice you need at least twice the mapped figure, plus enough frontage and access for both lots.

Not if it qualifies as exempt development: no more than 25 m² in area, a floor no higher than 1 m above ground, no more than 3 m high overall, behind the building line and at least 900 mm from the lot boundary. On a lot over 300 m², all decks and patios together are capped at 15% of the house's ground floor area. Outside those limits you need a CDC or a DA.

Up to 20 m² floor area in residential and most other zones, or up to 50 m² in rural zones (RU1–RU4, RU6, R5), provided it's no higher than 3 m, behind the building line and meets the boundary setbacks. Anything larger needs a CDC or DA.

Up to 1.8 m above existing ground level behind the building line, and up to 1.2 m forward of the building line or in a road setback. Masonry is limited to 1.2 m anywhere.