Underpinning a Semi-Detached House: The Shared Wall Explained

Footings, foundation walls, slabs, underpinning and framing in Toronto and the GTA.

Our own forms

No waiting on rental stock or another crew

Pour date in writing

The date we give you is the date we pour

WSIB cleared and insured

$5 million liability, certificates on request

Supply and install

Custom rebar and concrete at bulk pricing, one source

In short: you can underpin a semi-detached or row house, but the shared wall changes the job. The engineer designs how the party wall is supported, your neighbour should hear about it early, and the sections along that wall are dug with extra care.

Why a shared wall changes underpinning

In a semi or a row house, the foundation wall between you and your neighbour carries both houses. When you lower your basement, the new footing goes under that shared wall too. Done badly, it can crack or settle the house next door.

House over new basement, underpinning and basement lowering in Mississauga by Grand Structural Services — Underpinning and basement lowering by Grand Structural Services.

How it’s planned

The engineer’s design covers the party wall. It sets how deep to go, the section sizes and the order they’re dug, with the shared wall in mind.
A look at the neighbour’s side first. Photos and notes of their basement and walls before work starts protect both of you.
Short sections, never side by side. Each section is dug, poured and cured before the next one next to it is opened.
Engineer reviews on site. Especially along the shared wall.

Talking to your neighbour

Tell your neighbour before the permit is issued, not the day the crew arrives. Explain what’s being done, how long the noisy part will take and who to call with a concern. If the work needs access to their property, you need their permission.

Underpinning plumbing rough in, underpinning and basement lowering in Vaughan by Grand Structural Services — Underpinning and basement lowering by Grand Structural Services.

Can you underpin only your half?

Yes. Most semi-detached underpinning lowers only one side. The new footing under the shared wall supports it at the new depth, and the neighbour’s basement stays as it is.

Questions people ask

Do I need my neighbour’s permission to underpin?

The building permit is for your property. But if the work needs access to the neighbour’s land, or affects their side, you need to work it out with them first. Talk early.