Seiwa: What Defines Your Lot Boundary

Reviewed and updated 14-10-2026. This article previously said Seiwa decided that stratum height notations define lot boundaries. It did not: the Court applied the default boundary rule because the plan was silent, and held that the tiles and the membrane beneath them were common property. We have corrected the summary and removed a later case we could not verify.

It's the most basic question in strata: where does your lot end and where does common property begin? Seiwa is the NSW Court of Appeal decision that shows how the default boundary rule works when the strata plan doesn't say — and why tiles and waterproofing membranes are common property by default.

The starting position

Where a strata plan doesn't describe a boundary, the law supplies a default. Today it is section 6 of the Strata Schemes Development Act 2015 (NSW): the boundary is the inner surface of the walls (s6(1)(a)(i)), the upper surface of the floor and the under surface of the ceiling (s6(1)(a)(ii)). At the time of Seiwa the same rule was in section 5(2) of the 1973 Act.

The building's structure — wall structure, slab, screed, membrane, roof — sits outside those surfaces, so by default it is common property.

What the Court decided

• The plan did not define the lower boundary of the lot, so the default applied: the boundary was the upper surface of the floor as it was at registration.
• Tiles laid before the plan was registered were part of the floor. So the tiles and the waterproofing membrane beneath them were common property ([24]–[39]).
• The boundary is fixed at registration — it is "not ambulatory" and doesn't move when someone later re-tiles ([39]).
• The owner recovered lost rent, measured at the rent actually being paid ([45]–[47]).

The first-instance decision ([2006] NSWSC 1157) is also important: it held that the owners corporation's repair duty is strict ([3]), that once on notice delay and reasonable efforts are no defence ([14], [21]–[23]), and that an owner can't recover the cost of repairing common property themselves ([27]–[28]).

How it applies to townhouses and villas

The same default applies to townhouses and villas. A townhouse is not treated differently just because it is a townhouse — the question is always what the registered plan says.

• Plan silent: the slab, floor structure, screed and membranes — including in bathrooms, and including the floor between your own two storeys — are common property.
• Plan notation: a boundary described on the registered floor plan overrides the default (s6(1)(b)). Some townhouse plans include a notation that the lot includes the building structure; if so, the result is different.

What Seiwa is not authority for

Seiwa is not about roofs, financial hardship, or any difference between townhouses and apartments. It is about reading the plan and applying the default where the plan is silent.