An extension is never built in isolation. It has to join a house that may be decades old, built to an earlier code and settled into its site. Most of the issues raised at inspection happen right at that junction between new and existing work.
Footings, levels and movement
New footings sit on freshly excavated ground, while the old house has had years to settle. If the two parts are rigidly joined, cracking can appear along the junction. Engineers often specify an articulation joint or a particular connection detail to manage that movement. The certifier will compare the excavation and footing work with the engineer's design at the footing inspection.
Floor levels are another common trap. Matching the existing floor sounds simple, but a new slab may need to sit higher to keep the required clearance from the ground. Plan any steps between old and new floors early, because they affect stair and barrier rules.
Termites, damp and subfloor air
The termite management system around the house must stay continuous. When an extension cuts through an existing barrier or adds a new slab edge, the new and old systems need to be connected, with evidence provided to the certifier.
On older homes with timber floors, an extension can block the subfloor vents that keep the original floor dry. Moisture problems in the existing house are a frequent result. Plans should make clear how air will keep flowing under the floor, and how a damp proof course will be continued where new walls meet old ones.
Roofs, gutters and stormwater
Roof junctions are a source of leaks and defects. Box gutters, valleys and flashings at the join between old and new roofing need careful detailing and enough fall to drain. New downpipes must connect to the approved stormwater system, not discharge onto the ground beside the house. Stormwater connections are a mandatory inspection stage before they are covered, so leave trenches open until the certifier has seen them.
Light, air and fire near boundaries
An extension often covers existing windows. If a bedroom or living room loses the window it relied on for natural light or ventilation, the design must provide another source. Certifiers check this on the plans, but it is often missed when layouts change during construction.
If the new wall sits less than 900 mm from a side boundary, it must be built to the fire separation rules for external walls, and windows in that wall are restricted. Smoke alarms are also commonly required to be brought up to the current standard across the whole house when it is altered, so check the conditions of your approval.
What happens with the old parts of the house
The certifier assesses the approved work, not the whole house. That said, new work must not make the existing building less safe or less compliant. If earlier owners carried out unapproved work, it can complicate sale or future approvals. A building information certificate is the usual way to regularise it.
Making inspections run smoothly
A few habits help an extension pass each inspection:
- Keep the stamped approved plans and engineering on site.
- Book each inspection at least 48 hours ahead through our inspection booking page.
- Have certificates ready for termite protection, waterproofing and structural work.
- Raise any change to the design with the certifier first, rather than after it is built.
Our guide to Critical stage inspections: what we check and when lists each mandatory stage. Read about our alterations and additions certification, or to have plans checked before lodgement, see our plan assessment service.
Local help across NSW
CertifiBuild certifies Class 1 and Class 10 work in 39 council areas. Local rules on these council pages are a good place to start: Mosman Council, Randwick City Council, North Sydney Council, City of Sydney, Shellharbour City Council, Northern Beaches Council.
Suburb pages with local planning notes: Clifton Gardens, Clovelly, Crows Nest, Darlinghurst, Flinders, Freshwater, Georges Heights, Gerringong, Glenhaven, Grays Point. You can also browse every service area.
Sources
- External wall fire separation (Housing Provisions Part 9.2), ABCB
- Smoke alarm requirements for houses, ABCB Housing Provisions
- Home renovation under complying development, NSW Planning Portal