Can you build a home addition in winter?

Construction around Toronto doesn’t stop in November, and neither do the additions we build. Framers, roofers and concrete crews work through the winter all over the GTA, and an addition uses the same materials and the same Building Code in January as in July. What changes is the effort it takes to protect the work. Frozen ground, cold concrete, ice on the deck and short days all add steps that a July project doesn’t need.

So the real question isn’t whether an addition can be built in winter. It’s which stages make sense to do in the cold, which are better left for spring, and what the winter version costs in heat, tarps and time. Some families start digging in December because the timing suits them; others use the winter for the paperwork and break ground in April. We’re happy to plan either way, as long as the choice is made on purpose rather than by default.

This guide walks through what actually happens at each stage when the weather turns. For the bigger picture on shapes, connections and costs, see our home additions page.

The top layer of soil freezes in a GTA winter, and how deep depends on the cold snap, the snow cover and the soil. An excavator can break through a frozen crust without much trouble. The difficulty is what happens after the hole is open. The Ontario Building Code requires the bottom of an excavation to be kept from freezing for the entire construction period, because soil that freezes and thaws under a footing can heave and settle.

In practice, we dig, inspect and pour without long gaps, and keep the bottom covered between stages. Insulated blankets, straw, ground heaters or a heated enclosure keep the frost out. If frost does get in, the frozen material has to be dealt with before concrete goes down, not poured over.

The soil that comes out of the hole needs a plan too. A pile of excavated earth left in the yard freezes into hard lumps, and lumps of frozen backfill pushed against a new foundation settle when they thaw in spring. Where space allows, we cover the spoil or haul it away and bring clean fill back. On a tight Toronto lot there may be nowhere to keep a pile at all, so we plan that before the excavator arrives.

Fresh concrete can be ruined if it freezes before it has gained early strength, so the Building Code sets out what has to happen when it’s cold. When the air temperature is below 5°C, concrete has to be kept between 10°C and 25°C while it’s mixed and placed, and then held at 10°C or warmer for 72 hours after placing. No frozen material or ice is allowed in the mix.

On our winter sites you’ll see what that takes. The ready-mix supplier can heat the water and adjust the mix. Forms and reinforcing steel are cleared of snow and ice before the pour. Afterwards, the footings and walls disappear under insulated blankets or inside a tarp enclosure with a heater running, sometimes for several days. The concrete supplier and the engineer set the details for your foundation, and we follow them.

The footing forms are also an inspection point. In Toronto, footings are inspected when the formwork is complete and before the concrete is poured, so the pour date depends on the inspector’s visit as well as the forecast. A cold snap or a storm can push the pour back, and that’s far better than pouring into the wrong conditions.

If part of the foundation is concrete block rather than poured walls, the masonry has its own rule: mortar and masonry have to stay at 5°C or above while they’re laid and for at least 48 hours afterwards, and the top of unfinished masonry must be covered whenever work stops.

Once the foundation is in, framing can carry on in almost any weather. Our work day starts with clearing snow and ice from the floor deck, because nobody can work safely on it and meltwater soaks into the joints. Short days matter too. Around the winter solstice Toronto gets only about nine hours of daylight, so the schedule leans on good planning rather than long evenings.

The bigger risk is water getting trapped. Lumber and panels that sit in snow, or a deck that collects meltwater, have to dry out before the walls are closed with insulation and drywall. We’ve written a separate guide on framing through a GTA winter that covers moisture readings, swollen subfloor edges and safe temporary heat.

Roofing is the stage most affected by cold. Roofing membranes, adhesives and shingles all carry manufacturer limits on installation temperature, and some products won’t seal properly until warmer weather. On a winter addition, the roof may get a temporary waterproof layer first and the final roofing once conditions match the product instructions. Snow on a temporary cover is heavy, so we check and clear covers after every storm.

In winter, the order of work protects your house as much as the addition. The new structure goes up on its own first: floor, walls, roof, windows and the exterior wrap. Only when that shell is closed to the weather do we open the existing wall between old and new. Until then, your house keeps its own exterior wall and its own heat.

When the opening day comes, we put up a temporary wall inside to hold back dust and cold while the new rooms are still unfinished. It’s usually framed and sheathed rather than a sheet of plastic, and it may be insulated if the addition won’t be heated for a while. Doors and hallways that lead to the work area are sealed, and heating registers near the opening are closed so the furnace doesn’t pull construction dust through the ducts.

A second-storey addition is the case where the season matters most, because the roof comes off the occupied house for a period. If that stage falls in winter, the cover plan, the forecast and how quickly the new walls can be closed need to be settled before anyone lifts a shingle. Talk through the dates and the backup plan for a storm at the start, not the week it happens.

A winter addition needs heat long before it has its own heating system. Concrete needs it to cure, and drywall needs it too: the Building Code requires the space to be kept at 10°C or above for 48 hours before taping and finishing, and for at least 48 hours afterwards. Paint, adhesives and flooring products have their own temperature limits from the manufacturer.

The type of heater matters. A direct-fired propane or gas heater releases its combustion by-products straight into the space, including carbon monoxide and moisture. An indirect-fired heater sits outside or vents its exhaust outdoors and blows only clean warm air in. Electric heaters produce no combustion by-products at all. For drying out a new frame we prefer indirect or electric heat with some air movement, because a direct-fired heater adds water to the very space we’re trying to dry.

Temporary heaters on a construction site are serious equipment. In Ontario, only workers who hold a training certificate may operate a propane, gas or oil heater, and the heater needs fresh air, stable footing and distance from its fuel cylinder. Please don’t bring your own space heater or patio heater into the work area. Temporary heat is a real cost, and our estimates show it as its own line.

Every project is different, but this is how the stages of an addition tend to sort themselves around a GTA winter.

If you’d like to dig in spring, the calendar works backwards from that date, and the slow parts are usually approvals rather than construction. Design starts with a current survey and a clear brief.

In Toronto, the review clock for a house-scale permit only starts once the application is complete, and a complete application includes an issued zoning certificate, which the City calls a ZAP. It’s a detailed zoning review of your drawings before the permit, and its fee is credited against the permit fees. Once an application is complete, the City’s House Stream reviews it within 10 business days. That figure is the review standard for a complete package, not the time it takes to get there.

If the design needs a minor variance, the Committee of Adjustment holds a hearing first, and the permit can’t be issued until that’s decided. Neighbours are notified and can take part in the hearing. Since November 28, 2022, only the applicant, the Minister, specified persons and public bodies can appeal a decision. A variance can still take a while and isn’t guaranteed, which is exactly why we like to start it in winter, not in April.

Outside Toronto, each city runs its own zoning and permit process. Our location guides for Mississauga, Vaughan, Markham and the other GTA cities explain where to start.

If you want to break ground when the frost comes out, these are the things we suggest getting moving between now and March.

• Order a current survey if you don’t have one; the site plan, zoning check and permit drawings all rely on it.
• Ask your city’s building department for any old permit drawings or site plans on file for your house.
• Write a one-page brief: who the new space is for, what it has to hold and which rooms it should sit next to.
• For a second storey, have an engineer look at the existing walls and foundation before the design goes further.
• Get the zoning confirmed for the design, and file any minor variance as early as possible.
• Have the furnace, air conditioner and electrical panel assessed for the extra space.
• Choose windows and exterior materials so they can be ordered against the final drawings.
• Agree the construction start, the access route for the excavator and where materials will be stored.

These pages pick up where this guide leaves off.