What is involved in building a granny flat or secondary dwelling?

The terminology differs by state, secondary dwelling, granny flat, ancillary dwelling, and so does the treatment. What is consistent is that it is a dwelling and not a shed, so it carries the full weight of both approval systems.

What planning usually controls

• Maximum floor area, which is capped in most schemes.
• Whether the lot is large enough, since minimum site areas are common.
• Setbacks from boundaries and separation from the main dwelling.
• Total site coverage and private open space retained for both dwellings.
• Car parking, which is frequently the binding constraint on a small block.
• Whether it can be occupied by anyone or only by a related person, which some schemes restrict.
• Whether it can be separately titled or leased, which is usually the real commercial question.

Where the cost actually goes

• Services. Water, sewer, electricity and often gas have to be extended, and metering arrangements decided.
• Stormwater, which needs a legal point of discharge and often on-site detention.
• Sewer connections, which may need a build-over approval if a main crosses the site.
• Site works, since backyards are rarely level and access for machinery is often poor.
• Fire separation from the existing dwelling and the boundaries.