Gravity never takes a day off. On a sloping section, every downpour sends water across and through the ground towards the lowest point, and if a house or a retaining wall happens to be in the way, the consequences build up slowly and expensively.
Hillside properties are common across much of the North Island, from Wellington’s steep suburbs to the hillier parts of Auckland, Hawke’s Bay and Gisborne. They offer views, privacy and sun, but they also bring drainage and earth retention into play in ways a flat section rarely does.
Stormwater around the house
Roof water collected by spouting and downpipes should be piped to an approved outlet, typically the council stormwater network or, on some rural and semi-rural properties, a soak pit or tank. Downpipes that discharge onto the ground beside the house, or spill into a garden bed against the foundation, can saturate soil and contribute to subfloor dampness or ground movement.
Surface water is the other half of the picture. Paths, driveways and lawns should fall away from the building. A cut into the slope behind a house needs a way to intercept water before it reaches the walls, often a channel or sump at the base of the bank. Warning signs include water staining on the lower parts of walls, silt marks, ponding after rain and ground that stays damp and spongy for days.
Neighbouring land matters too. Water from properties further up the hill may cross the section by natural flow paths or through formal easements, and blocked or overloaded drains upstream can cause trouble downstream. The LIM sometimes records overland flow paths, and council drainage plans can show where public pipes run beneath private land.
Wastewater and the gully trap
Sewer pipes in older properties may be earthenware, which can crack or be infiltrated by tree roots. Slow drains, gurgling fixtures or repeated blockages are early clues. A visual inspection cannot see inside buried pipes, so where concerns arise, a drainlayer can run a camera through the lines to check their condition.
The gully trap, where waste pipes from the kitchen, laundry and bathroom discharge before entering the sewer, should sit high enough above surrounding ground that stormwater cannot flow into it, and low enough relative to the floor that a blocked sewer overflows outside rather than inside the house. It is a small detail, but one that inspectors routinely check.
Retaining walls
Retaining walls hold back soil where the ground has been cut or filled. Timber pole walls, concrete block walls, crib walls and gabion baskets are all common. Each fails in its own way: timber rots at ground level, block walls crack or lean, and any wall without proper drainage behind it can be pushed over by the build-up of water pressure in the soil.
Things to look for include leaning or bulging, cracks, rotten or split timber, soil washing out at the base, and missing or blocked weep holes and drainage outlets. A wall supporting a driveway, a building or another load, often described as a surcharge, is more critical than one holding back a garden bed.
Consent rules are relevant here. Lower walls that support no extra load are often exempt from building consent, but taller walls, and walls carrying a surcharge, generally need consent and engineering design. The LIM and the council property file should show whether significant walls on the site were consented and signed off.
Ownership can also be less obvious than it looks. A wall on or near a boundary may belong to one neighbour, both, or neither in any clearly documented way, and repairs to a shared wall can become a point of disagreement. A survey or title check helps clarify where the boundary actually runs.
Making sense of the findings
A general inspector can identify visible concerns, but structural assessment of a large retaining wall, or of a slope showing signs of movement, belongs with a geotechnical or structural engineer. Replacing a failed wall can be a significant expense, and work on steep sites often involves restricted access and specialist equipment.
Anyone buying on a slope should read the site and drainage sections of their building report with particular care, cross-check them against the LIM’s hazard information, and follow up any recommendation for specialist advice before the purchase becomes unconditional. Questions about past slips, instability or drainage complaints can also be put to the council and the vendor directly.