Granny Flat Guide: How to Build a Legal Secondary Dwelling in Australia

What is a Secondary Dwelling?

A secondary dwelling (granny flat) is a self-contained dwelling on the same lot as a principal dwelling. It can be attached, detached, or a conversion of an existing structure. Under NSW's SEPP (Housing) 2021, granny flats on lots of 450m² or more are permitted as complying development — one of the most permissive policies in Australia.

State Rules at a Glance

• 1 NSW: Lots 450m²+, max 60m² floor area, CDC pathway available
• 2 VIC: Secondary dwelling must be 'subservient' — typically under 60m², same lot as principal dwelling
• 3 QLD: Auxiliary unit allowed on residential lots, planning scheme requirements vary by LGA
• 4 WA: Survey strata subdivision may be easier than secondary dwelling in some LGAs
• 5 SA: Secondary dwelling permitted in most residential zones — check setbacks carefully

CDC vs DA for Granny Flats

In NSW, most standard granny flats can be approved via a Complying Development Certificate (CDC) through a private certifier in 10–20 days — much faster than a DA through council. Interstate, the process varies: check your state's planning legislation.

Design Considerations

• 1 Minimum ceiling height: 2.4m for habitable rooms
• 2 Minimum room sizes: bedroom 7m², combined living/kitchen/dining 17.5m²
• 3 Separate entry — not accessed through principal dwelling
• 4 Natural light and cross-ventilation in all habitable rooms
• 5 Laundry access — can be shared with principal dwelling or separate

Utilities and Metering

Separate metering for water and electricity is strongly recommended if you plan to rent the granny flat independently. Your network provider (electricity) and water authority can advise on costs. In most states, a separate connection point is required for a separately metered supply.

Build Costs

A standard 60m² 1-bedroom granny flat in Australia costs $120,000–$200,000 turnkey (including DA/CDC, design, construction, landscaping). Costs vary by state, site conditions, and finish level.