Can You Underpin a Semi-Detached House in Toronto?

Why semis are different from detached houses

Much of Toronto's older housing stock, from the Annex and Leslieville to Mimico and parts of Scarborough, is semi-detached. In a semi, the two houses share a party wall, and in many cases the two foundations sit side by side or even overlap slightly. When you excavate beneath your footing, you are working right next to your neighbour's foundation, so the engineering has to protect it as well as yours.

That is why a semi costs more than a detached home with the same basement. It is not that the work is fundamentally different. It is that the sequencing, shoring and monitoring are stricter, and the access is tighter.

Does my neighbour have to underpin too?

No. You can underpin your half without your neighbour doing anything. The engineer designs the work so that your lowered footing does not undermine theirs. What changes is coordination: your neighbour should hear about the project in writing before work starts, and your engineer and the City will tell you what notice or agreement is required for your property. Most disputes happen because someone hears jackhammers with no warning, so tell the neighbour early and give them a contact number.

How the work is done on a shared wall

1. 1**Engineer's site review and design.** The engineer investigates footing depth, soil and the condition of the party wall, then stamps drawings with a numbered pin sequence.
2. 2**Permit and notice.** The drawings go to the City for a building permit, and your neighbour is notified in writing.
3. 3**Alternating pins.** The crew excavates and pours small sections, usually about a metre wide, never two adjacent pins at once. Each pin cures before its neighbours are dug.
4. 4**Shoring along the shared wall.** Temporary supports carry the load while each section is open.
5. 5**New slab and drainage.** Once all pins are in, the floor is lowered and a new slab is poured. This is the best time to add waterproofing and weeping tile, since the walls are exposed anyway.
6. 6**Inspections.** The City and the engineer inspect at set stages before sections are backfilled.

Cost: what to expect for a semi

Basement underpinning in the GTA costs $35,000 to $140,000 for the structural phase depending on size and depth. A typical full-basement job on a detached house runs about $55,000 to $100,000+. A semi usually costs 15 to 25% more than a comparable detached house, which puts a typical semi at roughly $63,000 to $125,000+ before HST.

Permit fees are not in these figures. They are billed at cost on top of the price, as a separate line or invoiced when the amount is known. HST of 13% is charged on the whole pre-tax price.

Timeline

• **4–8 weeks** for engineering and permit processing before digging starts.
• **8–16 weeks** for the active underpinning phase on a typical GTA home.
• Add time for finishing if you are building a legal basement apartment, since that brings extra requirements such as ceiling height and egress.

Is it worth it in a semi?

Often, yes. Semi-detached homes in Toronto are among the most in-demand property types, and there is little room to expand sideways, so lowering the basement is the main way to add usable space. Our guide to underpinning cost covers how it affects resale value, and the bench footing option can reduce cost where you can live with narrower floor space along the walls.

FAQs about underpinning a semi

Planning to lower the basement in a semi?

We start with a site visit, look at your footing depth and the party wall, and give you a written scope. Engineer coordination, permits and a clear timeline are part of the plan from day one. Free, no obligation, $0 down. Call (647) 254-0877.