Three storey home certification

Can you get a fast-track approval?

The Housing Code (the state rules for a quicker CDC approval) applies only to new homes with two storeys or fewer. Alongside the 8.5 m height limit it caps storeys at two. A design under 8.5 m can therefore still miss out simply because it has three storeys.

Whether a lower level counts as a storey depends on how it sits against the ground and on planning definitions. We check the sections early, before plans are final, so you know your pathway.

Where a CDC cannot be used, a development application (DA) goes to council, which assesses it under its local planning rules, including any mapped height limit. After approval, the construction certificate plans must be consistent with the consent.

Building on a slope

Three levels are common on steep lots, with the lower floor cut into the slope. Sites with soft or loose soils, landslip risk or mine subsidence are rated class P. An engineer must design the footings and slabs, usually with a soil report.

Excavating for a lower level also calls for temporary support, and drainage behind any retaining wall. Both should be specified by the engineer, along with the sequence of excavation and backfill, so the wall can be inspected while still exposed.

Smoke alarms and fall heights

Every storey needs a smoke alarm, even levels with no bedrooms. On bedroom levels, it goes in the hallway serving the bedrooms. Multiple alarms must be linked, mains powered where mains power is connected, and meet AS 3786.

A third level also means bigger falls. Where someone could fall more than 4 m, a balustrade must not have horizontal rails between 150 mm and 760 mm above the floor that a child could climb.

Concrete floors and loads

A suspended concrete floor between levels needs its own structural design, usually to AS 3600, with the steel checked before the pour. Loads from the top floor must be traced down to the footings. Engineering drawings should show beams, columns, propping and any transfer beams, so inspections can be checked against them.

These figures are taken from NCC 2022, the edition currently in force in NSW. NCC 2025 takes effect in NSW from 1 May 2027, so projects that run for a long time should check which edition applies.

Draining a level cut into the slope

A lower floor built into a hillside sits against damp ground on at least one side. Where subsoil drains are used, the NCC (National Construction Code) Housing Provisions require them to have an even fall of at least 1 in 300, to discharge into a silt pit or sump outside the building, and to allow for cleaning and maintenance.

The outlet from the silt pit into the sealed drainage line must sit at least 50 mm below the inlet, so silt settles out before water moves on. Under any suspended floor, the ground must be shaped so water cannot pond beneath the house, with the area under the building kept above the finished ground outside.

Ask the engineer and the stormwater designer to show the drain behind each retaining wall, its outlet and the pit together on one drawing. These parts are buried early in the job and are hard to check later, so take dated photos of them before any backfill goes in.

Frame design beyond standard house limits

Timber and steel frames and roof trusses are often designed with software built around the Housing Provisions. The NCC requires that software to follow the ABCB protocol for structural software (ABCB stands for Australian Building Codes Board). That requirement only covers buildings within set limits. Those limits are eaves no more than 6 m above the ground, a roof no more than 8.5 m above the ground, a width of 16 m or less, a length no more than five times the width, and a roof pitch of 35 degrees or less.

A home on three levels, especially on a slope, can go past the eaves or roof height limit on its low side. In that case it is sensible to have the structural engineer confirm the frame and truss design instead of depending only on standard software output.

Before the frame is ordered, check that the engineer's drawings and the supplier's layouts agree on beam sizes, tie-downs and the path that loads take down to the footings. Mismatches found at the frame inspection are far harder to fix.

Changing a design after council approval

Most three storey homes are approved by development application (DA), so later design changes go back to council as a modification under section 4.55 of the Environmental Planning and Assessment Act 1979. There are three kinds: correcting a minor error or a change with no environmental impact, a change with minimal impact, and other changes with a greater impact.

Each kind is limited to a development that remains substantially the same as the one approved. The comparison is made with the original consent, not the most recently modified version, and council can weigh up both numbers, such as height and floor area, and qualities, such as amenity for neighbours.

On a three level home, raising a floor level or extending a top floor terrace can affect height, overlooking and bulk all at once. Work out whether a change needs a modification before the construction certificate plans are finalised, rather than once the frame is up.

Basements, lower levels and floor area

If a design with a lower level qualifies for the Housing Code, how that level is used affects the floor area count. Gross floor area includes habitable rooms in a basement or attic, but leaves out storage basements, areas for vehicle access, garbage and services, and one car space with its access.

A lower level fitted out as a rumpus room or bedroom therefore adds to the total, while the same space kept for parking, storage or plant may not. Label the use of every room clearly on the plans, because turning storage into living space later can affect compliance with the approval.