Double storey home certification

Why upper floors step in

Taller walls need bigger side setbacks. For lots wider than 10 m and no wider than 18 m, walls up to 4.5 m high need 900 mm. Beyond 4.5 m, add one quarter of the additional height, so a 7 m wall needs about 1.53 m. For lots 6 to 10 m wide, the increase only begins above 5.5 m.

Rear setbacks also jump. On most lots from 250 m² to 900 m², parts of the house between 4.5 m and 8.5 m high need an 8 m rear setback. The lower level only needs 3 m. That is why upper floors are often set in from the walls below.

Raised balconies and decks

Where a lot is wider than 10 m, side and rear balconies, decks and terraces are height limited. If the lot exceeds 300 m², the floor may sit up to 2 m above ground within 3 m of the boundary, up to 3 m at 3 to 6 m, and up to 4 m beyond 6 m. The code does not allow them at all on lots no larger than 300 m².

Where one sits closer than 6 m to a rear or side boundary and over 2 m above ground, its area is capped at 12 m². Privacy screens may also be needed, which we check on the elevations.

Stairs, balustrades and handrails

Each flight needs 2 to 18 risers, so a tall floor-to-floor height may need a landing. Riser and tread sizes can vary by no more than 5 mm from one step to the next, and 10 mm across the flight. Open risers are fine, but the gap must not let a 125 mm ball through.

A balustrade is required anywhere a fall of 1 m or more is possible. Minimum heights are 865 mm on stairs and 1 m on landings, balconies and decks. Every flight also needs a continuous handrail, 865 mm high or more, on one side at least.

Upstairs bedroom windows

Where a bedroom floor sits 2 m or higher above the outside ground, limit or screen the openable part of the window so a 125 mm ball cannot get through. That restrictor or screen has to withstand 250 N pushing outwards. Any device that can be removed or overridden must have a child-resistant release. None of this applies when the openable part is 1.7 m or higher above the floor.

How the height limit is measured

The Housing Code sets 8.5 m as the maximum height for a new house. Height is measured from the existing ground level, meaning the ground as it was before any cut or fill. Digging the site down does not give a two storey design any extra height.

Where the lot slopes, the gap between ground and roof is largest on the low side, so that is usually where the limit bites. A detail survey with spot levels across the whole footprint lets the designer draw the 8.5 m line on every elevation and section, and shows early whether a lower roof pitch or a stepped floor is needed.

An attic is not counted as a storey under the code. A loft room built inside the roof can therefore sit above the upper floor without turning the home into a three storey design, as long as the overall height still fits. Habitable attic rooms do count towards gross floor area, so add them to the floor area calculation.

Bathrooms on a framed upper floor

Upstairs bathrooms usually sit on timber, plywood or particleboard flooring rather than a concrete slab, and that changes the waterproofing rules. Under the NCC (National Construction Code) Housing Provisions, a bathroom floor on a timber based substrate must be waterproof across the room. On a concrete or fibre cement sheet floor, the same area only needs to be water resistant.

Inside the shower, the floor and any hob or step-down must be waterproof, and the walls must be waterproof to at least 1800 mm above the floor substrate. The shower floor must fall to the waste at no flatter than 1:80 and no steeper than 1:50. Wall to floor junctions must be waterproof, and any flashing used there needs a horizontal leg of at least 40 mm.

Wet area waterproofing is a critical stage, so the principal certifier (PC) must inspect it before tiles or screeds cover it. On an upper floor, a failed membrane shows up as a stained ceiling below, so keep the installer's details and photos of the membrane for the inspection.

One frame inspection, two levels

The PC must inspect the framework before it is covered. That inspection happens once wall and roof cladding, wiring and plumbing rough-in, and window and door frames are installed, but before insulation or internal linings go on.

On a two storey home, this one visit covers a lot of structure: the upper floor joists or floor trusses, any steel beams carrying upper walls, bracing and tie-downs on both levels, and the roof frame. Each item is compared with the engineering and the frame and truss layouts that support the approval, so keep current copies on site and make sure late changes have been signed off by the designer.

A visit by the structural engineer does not replace the certifier's inspection. If linings go on before the frame is inspected, the missed stage can delay the occupation certificate (OC).

Walls and eaves close to a side boundary

Narrow lots can bring a two storey home close to its side boundaries. Under the Housing Provisions, fire protection is needed for any section of an outer wall that sits within 900 mm of the boundary with a neighbouring lot, or within 1.8 m of a separate building on your lot. Ways to achieve it include solid masonry of 90 mm or more, a masonry veneer whose outer leaf is at least 90 mm, or a system rated FRL (fire resistance level) 60/60/60 when tested from the outside.

That wall has to rise from the footings or slab all the way to a non-combustible roof covering or eaves lining above. Only the part of the wall inside the 900 mm zone needs protection, so an upper level that steps back beyond that line may not need it.

Openings in a protected wall are tightly limited, with small allowances for windows to bathrooms, laundries and toilets that are at least 600 mm from the boundary. Eaves with non-combustible roofing and lining may come no closer than 450 mm to the boundary, and a deck is not a permitted encroachment inside the 900 mm zone.