Toronto is a city of semis and Victorian row houses, which means most underpinning here happens next to somebody else's foundation, and often under a wall that is shared. That single fact changes the engineering, the sequence, the timeline and the conversation you need to have with your neighbour before anything starts.
If you own a detached house on a wide lot, this page is background. If you own a semi, a row house or a townhouse, this is the part of the project most likely to surprise you.
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The party wall is the whole issue
In a semi-detached or row house, the wall between you and your neighbour is typically a shared structural wall sitting on a shared footing. When you lower your basement, you are excavating beside and beneath a foundation that is holding up two houses, not one.
That has three practical consequences.
The engineer's scope gets wider. The stamped drawings have to address not just your house but the effect on the adjoining structure. Section widths are usually narrower, the sequence is more conservative, and the engineer may specify monitoring during the work.
The sequence gets slower. More sections, smaller pours, more curing time between them. A detached house that might be done in four weeks of on-site work can take six or more as a row house.
Your neighbour becomes a participant whether they want to be or not. Even where there is no legal requirement to obtain their consent for work entirely on your own property, work adjacent to a shared foundation is not something to spring on someone. Their cooperation is often needed for access, and their goodwill is worth more than it costs.
The engineering next to a party wall is not harder because the concrete is different. It is harder because a mistake has two addresses.
Talk to your neighbour first, and do it properly
The single best predictor of a smooth row-house underpinning is a neighbour who was told early, in person, with the drawings in hand. The single best predictor of a stopped project is a neighbour who found out from the noise.
What to cover:
- What is being done, and where. Show the drawings. Most anxiety here comes from imagining something worse than what is planned.
- The sequence, and why it means their house is never unsupported. This is genuinely reassuring and it is genuinely true, so say it.
- Timeline, including the noisy part. Be specific. "Roughly six weeks of excavation and pours, weekdays, starting in September" is a fact people can plan around.
- Whether you need access to their side for scaffolding, monitoring points, or restoration.
- A pre-construction condition survey. This one matters more than the rest.
Get a condition survey before anything is dug
A pre-construction condition survey is a dated photographic and written record of the existing state of the adjoining property: existing cracks, existing settlement, existing everything. It is usually inexpensive relative to the project and it protects both households.
It protects your neighbour because it establishes what was there before, so genuine new damage is provable.
It protects you because a hairline crack that has been in their basement since 1974 cannot be attributed to your excavation once it is photographed and dated.
Skipping the condition survey is the most expensive small saving in this entire trade. Without one, every pre-existing crack in the adjoining house becomes an argument you cannot win.
What changes in the scope and the price
Row houses and semis are not simply smaller versions of detached jobs. Several things move.
Access. In a detached house on a decent lot, spoil goes out the side to a bin on the driveway. In a downtown row house with no side yard, every wheelbarrow of soil comes out through the house, or through the front, by hand. Excavation and disposal costs rise accordingly, and this is a common reason two quotes for identically sized basements differ.
Section widths. Narrower sections next to a party wall mean more pours, more mobilisations, more days.
Monitoring. Some engineers specify movement monitoring on the adjoining structure during the work. That is a line item, and it is a reasonable one.
Shoring and support. Where the party wall footing is shallow or the soil is poor, temporary support may be specified.
Restoration. A shared walkway, a shared drive, or a shared fence dug through has to go back, and it is worth being explicit about whose it is and who restores it.
Perimeter measurement. This is a genuine cost advantage of a semi and it is worth noticing: a semi has fewer exterior walls than a detached house of the same floor area, and a mid-row house fewer still. Since underpinning is priced per linear foot of foundation wall, the same square footage often costs less to underpin in a row house than in a detached one, before the access penalty is added back.
Can you underpin only your half?
Yes, and it is common. Underpinning is routinely done on one side of a party wall while the neighbour's basement stays at its original depth. The engineer designs for the step in level and the resulting load path.
What you should understand:
- The design is different from underpinning both sides, and it needs to be designed that way from the beginning, not adapted mid-project.
- Your neighbour underpinning later is easier than doing it at the same time, but it is easier still if your engineer knows it may happen and drawings anticipate it.
- Doing both at once, where both households want the work, is genuinely cheaper per house. One mobilisation, one engineering exercise, one permit process, one disruption. If your neighbour has ever mentioned wanting a taller basement, it is worth raising.
The Toronto row house pattern
Much of Toronto's older row and semi stock shares a set of traits that shape these projects:
- Shallow rubble or brick footings, which is precisely why the basements are short in the first place
- Basements originally intended as cellars, at 5 feet 10 inches to 6 feet 6 inches
- No side access, which drives up excavation cost in the downtown core
- Old drainage, sometimes clay, frequently at the end of its life, and worth replacing while the trench is open
- Party wall footings that are not always where the drawings assume, which is one more reason the contingency line exists
None of this makes the work unusual. It is the standard condition here, and any Toronto crew that does this work regularly will have seen all of it. It does mean that a contractor whose experience is entirely suburban detached houses is learning something on your street.
Common questions
Do I need my neighbour's permission to underpin a semi-detached house?
Work entirely within your own property generally does not require their consent, but the practical answer is that you need their cooperation for access, for the condition survey, and for the project to go smoothly. Whether any formal notice or agreement is required for your specific situation is a question for your designer and, if the party wall is directly affected, a lawyer.
Is underpinning a semi more expensive than a detached house?
Per linear foot it is usually somewhat higher because of narrower sections, slower sequencing and often harder access. Per square foot of basement gained it can be lower, because a semi has less exterior wall for the same floor area. Compare the total, not the rate.
Will underpinning damage my neighbour's house?
Properly engineered and sequenced, it should not. The alternating section method exists precisely so that no part of the foundation is ever unsupported. This is also why the condition survey and, where specified, monitoring belong in the scope.
Can my neighbour stop the project?
They cannot generally stop permitted work on your own property, but a dispute can create delay, cost and years of unpleasant proximity. Early, specific communication is not a courtesy here, it is project management.
What if my neighbour already underpinned their half?
That usually makes yours simpler, because the adjoining foundation is already deeper. Your engineer will want to know what was done and to what depth, so ask your neighbour for their drawings.
Should we underpin at the same time?
If you both want it, yes. Shared mobilisation, shared engineering, one permit exercise and one period of disruption instead of two. It is meaningfully cheaper per household.
Get a real range
Not a brochure. Tell us the basics and we will come back with numbers that reflect your actual foundation.