Building: Two new double storey dwellings with garages on one lot, to be subdivided (Class 1a and 10a)
Location: South east Melbourne
NCC edition: NCC 2022, Volume Two and Housing Provisions
Area of NCC requirements: H3P1 fire separation (Housing Provisions 9.2.1, 9.2.3); H2P1, H2P2, H2P3 (Housing Provisions 3.3.3(b) and (c))
Raised by: The relevant building surveyor, in a request for further information
Year: 2026
The challenge
Three departures on one tight site. A first floor bedroom wall in one unit was closer than 1.8 m to the other unit. One unit's suspended timber floor sat less than 150 mm above the driveway, with subfloor ground below the paving. Next door, the other unit's garage floor was about 300 mm higher than that same living room, so the garage slab retains soil against the lower dwelling.
The performance solution
Expert judgement and comparison with the Deemed to Satisfy Provisions, with the hazards set out using the Australian Fire Engineering Guidelines approach.
Fire separation. The lot is being subdivided, and the Housing Provisions apply the 900 mm boundary rule as if the new boundary already exists. The 1.8 m rule is really two 900 mm setbacks. The bedroom wall is more than 900 mm from the future boundary, so the fire risk is no greater than a compliant house next to any boundary.
Floor below the driveway. The subfloor wall is filled solid and tanked on the driveway side with a nominated below ground sheet membrane over a render substrate, terminated above the paving. There is a damp proof course to the bearers, and a trafficable grated channel drain along the driveway edge. The subfloor ground falls to a socked agricultural drain, with vents only in the other walls.
Garage above the dwelling. The garage edge rib is deepened to retain the subgrade, and a hob is cast with the slab 100 mm above the garage floor. The garage floor falls away from the shared wall. The construction joint has dowels, a lapped under slab membrane and a hydrophilic waterstop. The same membrane wraps the lower face, and the work is sequenced so it is inspected before the dwelling is built against it.
Conditions
Amended architectural sections, a further structural design certificate for the retaining elements and waterstop, an amended stormwater plan with plumbing certificate, an installer's statement and photos before any membrane is covered, and an owner obligation to keep drains clear and not build up soil against the walls.
Outcome
Final report issued September 2026 and revised November 2026 to answer the building surveyor's comments. Successfully accepted by the relevant building surveyor.
Lesson for unit developers
Level changes between units on a sloping lot create waterproofing problems that a membrane alone will not solve. Get the engineer, drainage designer and building surveyor to agree the junction details before construction drawings are finished.
Related: Performance solution report | Performance solutions Victoria
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