Building On The Boundary And Party Walls

On a narrow Sydney site, building to the boundary is usually the only way to get the floor area. It also means the neighbour's building becomes part of your engineering problem.

What is actually on the boundary

The first question is what exists, and it is answered by survey rather than assumption.

A genuine party wall, shared and straddling the boundary, common in older terraces and semis. Both owners have rights in it and neither can do as they please.

The neighbour's wall, wholly on their land, with your building proposed against it. You have no rights in that wall.

Your own wall on your land, built tight to the boundary. Cleanest position, and it requires the whole wall including footings to sit inside your title.

Nothing, which is the simplest case and the least common in established suburbs.

A boundary survey by a registered surveyor is the starting point. Title boundaries and the position of built structures frequently disagree, sometimes by a surprising amount, and finding that out after the piling is set out is expensive.

The footing problem

The wall can sit on the boundary. The footing under it generally wants to be wider than the wall, which means it wants to cross the line.

The usual solutions are an eccentric footing, loaded off centre and designed for the resulting moment, or a strip footing turned to run along the boundary and tied back, or piles positioned inside the line with a capping beam.

Each costs more than a conventional pad. Discovering the need late is what turns it into a variation.

Excavation next to their building

This is where most of the risk sits.

Excavating below the level of a neighbour's footing removes the support their footing relies on. Depending on the depth and the ground, that is either a routine engineering exercise or a serious one.

There are legal obligations around supporting adjoining land and structures, and they are not discretionary. Practically that means a designed retention system, and frequently [underpinning](/blog/underpinning-adjoining-structures-explained) of their footing so it is founded below your excavation before you dig.

The [excavation near existing structures](/blog/excavation-near-existing-structures-best-practices) approach applies in full, and the closer and deeper the excavation, the more it matters.

Dilapidation and monitoring

A [dilapidation report](/blog/importance-of-dilapidation-reports) on every adjoining property before anything starts. Photographed, dated, and issued.

It protects the neighbour with a genuine claim and it protects you against one that is not, and without it the argument is unwinnable in either direction.

Then monitoring during the works: survey targets on their building, read at an agreed frequency, against trigger levels set in advance. Where [vibration](/blog/vibration-monitoring-on-construction-sites) is likely from piling, rock breaking or demolition, vibration monitoring runs alongside it.

Access you do not have

You will almost certainly need to go onto the neighbour's land at some point: to survey, to underpin, to build the far face of a wall, to install flashing.

You do not have that right automatically. It is negotiated, and it should be negotiated early and in writing, because a refusal changes the design rather than delaying it.

A wall that can only be built with access from next door, on a site where access has been refused, is a wall that has to be redesigned to be built entirely from your side.

Fire and acoustics on the boundary

A wall on a boundary generally carries a fire resistance requirement, and in residential work an acoustic one as well.

Both affect thickness and construction, which affects the footing, which affects the boundary geometry. They are not finishes decisions bolted on later, and on a shared wall they interact with what is already there.

Getting the sequence right

Survey, then investigate the ground, then establish what the neighbour's structure actually is and how it is founded, then design, then agree access and support arrangements, then start.

Reversing any of those steps is how boundary work generates disputes.

ACSES provides structural engineering and geotechnical engineering for boundary and adjoining structure work across Sydney. Talk to us.