Granny flat designs that work on a narrow Sydney block

On a narrow block the granny flat design is decided by arithmetic before anyone thinks about style. Work out your frontage, subtract the side setbacks, and what remains is the widest building you can put there. On a typical 450 m² to 900 m² lot with 0.9 m side setbacks, a 12 m frontage leaves about 10.2 m of buildable width.

That number, and the rear setback, decide almost everything else.

Do this before you look at a single floor plan.

Buildable width = frontage − side setback − side setback

Note what happens as the lot gets bigger. The minimum frontage rises by 3 m at each step, but the side setbacks rise too, so the buildable width grows more slowly than the frontage does. If your frontage is wider than the minimum, add the difference to the buildable width.

Then subtract from the depth:

• The rear setback for your lot size band, which is 3 m on lots between 450 m² and 900 m²
• Anything above 3.8 m in height adds 3 times the extra height to that rear setback, up to a maximum of 8 m on that band
• Your existing house and its required separation
• Access to the granny flat
• Required landscaped area, 20% to 45% depending on lot size, with at least half behind the building line

What remains is the building envelope. The design has to fit inside it.

Keep the building under 3.8 m tall.

Height above 3.8 m increases both side and rear setbacks. The rear loading is 3 times the extra height. So a building 4.8 m tall on a 600 m² block needs 6 m at the rear instead of 3 m, and a slightly larger side setback as well.

On a tight block that one metre of height can cost you the entire project. A single storey design with a low roof pitch, kept under 3.8 m, is usually the difference between fitting and not fitting.

Rooms arranged front to back along the length of the building rather than side to side. Usually living at one end, bedrooms at the other, with the bathroom and kitchen in the middle where they can share plumbing.

Suits envelopes that are narrow and deep, which is most narrow Sydney blocks.

Rather than entering into the living room, a short entry along the side wall leads into the space. Costs a little floor area and buys privacy and a place for coats and shoes, which small dwellings badly need.

On a narrow block your windows are close to a boundary and often looking at a fence. Corner windows draw light from 2 directions. Higher sills bring in light while keeping sightlines away from the neighbour.

Remember the privacy screen rules apply to habitable room windows within 3 m of a side or rear boundary where the floor level is more than 1 m above ground. Bedroom windows of 2 m² or less are exempt.

A swinging door needs about a square metre of clear floor to operate. Across 4 or 5 doorways in a 60 m² dwelling, that is real space. Cavity sliders recover it.

Built-in wardrobes, a study nook, bench seating with storage under it. In a small dwelling, joinery does the work that spare rooms do in a big one.

When the envelope will not fit everything, this is the usual order of sacrifice.

What not to give up: storage, natural light, and privacy from the main house. Those 3 are what make a small dwelling feel liveable or unbearable.

• Eaves. They project beyond the wall and count toward the setback in most cases. A design that fits to the millimetre on paper may not once eaves are added.
• Access for construction. If a truck and excavator cannot get down the side, construction costs rise.
• Where the sewer runs. You generally cannot build over a sewer main without approval.
• Trees and their protection zones. These can be larger than the visible canopy.
• Existing structures. A shed or an old slab may be consuming the envelope you need.

There are still options.

• A smaller granny flat. A well-designed 40 m² one bedroom is a real home. It is worth more than a 60 m² design that cannot be built.
• A development application. Council assesses on merit and can approve what the fixed checklist cannot.
• An internal conversion. A secondary dwelling within the existing house has different and simpler requirements, with no external alterations beyond one added entrance.