How big can a granny flat be?

As big as the cap where you are, and the council or the state sets the cap. Almost every scheme limits a granny flat’s floor area, and some step the limit with the lot’s size or the zone, so the same flat can fit on one block and not on the next. The cap is the start of the answer. A flat under it still has to leave the lot enough open space, parking and distance between the dwellings, and on a smaller lot those run out before the floor area does.

What counts towards the floor area?

The council’s definition decides. It always takes in the rooms, and depending on the wording it can also take in a deck or verandah under the flat’s roof, an attached carport or a store. The same plan can measure differently under another council’s definition, so the council’s wording matters more than the brochure’s.

What else limits the size?

The lot. A granny flat is assessed against the whole block rather than its own footprint, so the site cover of both dwellings together, the private open space left over and its narrowest width, a parking space that fits without blocking the existing one, and the distance from the house all bear on how big the flat can be. Site cover is where many plans come unstuck, because the house, any patio and the new flat are added together.

Who decides?

The council, or the state where it runs its own approval path for second dwellings, decides whether the flat can go on the lot and at what size, and a certifier or the council gives the building approval. The water utility comes in where the flat’s footings or its sewer connection go near a main.

What goes on the site plan?

It shows the flat at its real size where it will sit, with its floor area printed and its distance to the house and to every boundary. It marks the private open space and measures it, places the parking, and totals the site cover across both dwellings. With the floor area on the sheet, the cap becomes a yes or a no rather than a guess.

Reading it for your own lot

RapidSiteplan reads the lot from its address in a minute: the boundaries, the existing house traced from the aerial photo, the overlays, and the sewer, water and drainage the new footings have to clear. Place the flat and the sheet counts the site cover of both dwellings and dimensions the distance between them as you move it. The granny flats page shows the sheet, the post on whether a granny flat fits your block covers the zone and the lot, and the council pages for every state show what each council maps. Start with the address.

Common questions

How big can a granny flat be? As big as the floor area cap where you are, which the council or the state sets and some step with the lot’s size or the zone. The flat still has to leave room for open space, parking and the distance between the dwellings, which the council reads across the whole lot.