Designing a Custom Home in Ontario: 7 Decisions That Set the Cost

First, who draws it, and does it matter?

In Ontario a house that falls under Part 9 of the Building Code (most detached homes, up to three storeys and 600 square metres) does not need an architect by law. It does need permit drawings prepared by someone qualified. Most people who draw houses for a living carry a Building Code Identification Number, a BCIN, which the province issues after exams. The Code also lets you draw your own house. Division C, Article 3.2.5.1 exempts the owner of a detached, semi-detached, town or row house with no more than two dwelling units from the designer qualification rules, and Article 3.2.4.1 exempts an owner from registration for a building they own. There is no size limit and no rule that you have to live in it. What does not change is the standard: your drawings have to meet every part of the Code, the truss and floor-joist drawings still come from the manufacturer’s engineer, anything outside the Part 9 tables still needs an engineer, and most municipalities still want a heat loss calculation and an energy compliance form with the application. You sign the Schedule 1 form as the designer and you own every mistake on it. I have seen owner-drawn houses pass first time, and I have seen the building department send one back four times. The difference was never the drafting software.

Whether you use an architect, a BCIN designer or a design-build firm matters less than one thing: does the person know what the lines on the page cost to build? A beautiful drawing with four roof valleys, a cantilevered bedroom and a 14-foot great room is a beautiful drawing. It is also a budget with no number attached yet. I have priced a lot of those and the conversation that follows is never fun.

Read should I hire an architect before meeting with builders for the order that saves money (builder first, usually), and what permit drawings in Ontario have to include so you know what you are paying a designer for. The provincial rules on who may design what are on the Ontario Building Code page at ontario.ca, and the Ontario Association of Architects has a directory if your project needs one.

Decision 1: Shape. Every corner costs money, every valley costs more

A rectangle is the cheapest shape to build. Each jog in the foundation adds two corners, more formwork, more footing, more rebar, more labour and, later, more siding trim and more flashing. Each extra roof plane adds a valley or a hip, and valleys are where roofs leak when somebody gets lazy with the underlayment.

That does not mean your house has to be a shoebox. It means you should know what you are paying for. A simple two-storey rectangle with a good porch and honest proportions reads as a far more expensive house than a sprawling one-storey with six bump-outs, and it costs less per square foot to build and to heat. Ask your designer for the foundation plan early and count the corners. Eight is a house. Twenty is a hobby.

Decision 2: Size. The square footage trap

Everyone starts at a number of square feet, because that is how real estate listings talk. Builders do not price that way, and the reasons are in why cost per square foot in Ontario is misleading. A 2,000 square foot house with a walkout, a four-car garage and a big kitchen costs more than a plain 2,400 square foot house.

The honest way to size a house is to list the rooms you will use every week, give each one a real dimension, and add circulation. Then look at what is left: the formal dining room nobody eats in, the second living room, the bedroom that is really a closet with ambitions. Making a smaller plan work harder is the cheapest design move there is, because every square foot you do not build is a square foot you do not finish, heat, insure or clean.

Run the first number through the custom home building calculator before you draw anything. It is free, it is blunt, and it has talked more than one family out of a third storey.

Decision 3: One storey or two

For the same floor area, a two-storey house has half the foundation and half the roof of a bungalow. Those are the two most expensive parts of the shell, so two storeys is usually cheaper to build per square foot. The bungalow wins on something else: you can live in it at 80 without a stair lift, and that matters to more of my clients every year.

The compromise that works for a lot of people in Ontario is a bungalow with a usable basement, or a one-and-a-half storey with the main bedroom on the ground floor. If aging in place is on your mind, read our accessible design guide and the forever home features worth including before you lock the plan. Widening a hallway on paper costs nothing. Widening it after drywall costs a renovation.

If two or three generations are moving in together, that changes the plan more than any other single choice; see multi-generational home design for the layouts that actually keep the peace.

Decision 4: Foundation, walkout and the lot you are on

A full basement in Ontario is close to free square footage compared with the floors above it, because you have to dig below frost anyway. A walkout basement on a sloping lot gives you a bright lower floor and a second entrance, and it also gives you a taller, engineered rear wall, more excavation, retaining walls and grading. How sloped lots raise building costs walks through the numbers.

Decide the foundation system at the same time as the plan, not after. ICF (insulated concrete form) foundations and walls change the wall thickness, the window details, and the heating design, so the plan should be drawn for them from the start; ICF house plans explains what changes. For the lot itself, soil, water table and bedrock decide what the foundation costs long before the designer does; if you have not bought yet, start at buying a lot in Ontario.

Decision 5: Ceiling heights and windows

Nine-foot ceilings on the main floor are standard now and cost little extra. Ten feet, or a two-storey great room, costs more than people expect: taller studs or ICF courses, taller windows, more siding and drywall, bigger heating loads, and scaffolding for every trade that has to reach the ceiling. It also makes the room louder. I like a tall room as much as anyone. I just want you to buy it on purpose.

Windows are the single biggest line that swings with design. Big south glass is wonderful in January and a greenhouse in July unless the overhangs are designed for it. Count the windows on the plan, note the big ones, and ask what glass they are. Every bedroom window also has to meet the Ontario egress window rules, which is a cheap fix on paper and an expensive one after the wall is framed.

Decision 6: The mechanical room and the systems that live in it

The least glamorous room in the house decides how comfortable the house is and what it costs to run. Put the mechanical room where the duct or pipe runs are short, with a real door, a floor drain, and space to replace a water heater without removing a wall. Decide the heating system on the plan, not in the field: a furnace needs chases for ducts, radiant floors need a thicker floor assembly and a boiler, heat pumps need a place for the outdoor unit that is not under your bedroom window. Radiant versus forced air is the comparison to read before you pick.

Whatever you choose, the equipment should be sized from a heat loss calculation of your actual plan, not from square footage. Ontario requires it for the permit in most municipalities now, and it is the one piece of engineering that pays for itself in the first winter. OntarioHeatLoss.ca does the stamped version if your designer does not.

Decision 7: Finish level, and when to pick it

Finishes are where people think the money goes, and they are wrong, but only by a little. Kitchen, bathrooms, flooring, trim and exterior cladding can swing a build by a lot, and the swing is almost all choice, not necessity. The mistake is not picking expensive things; it is picking them late. A kitchen chosen after framing means moving plumbing and electrical, which means change orders, which means the builder’s price was never your price.

Use the kitchen cabinet guide and the bathroom design checklist while the plan is still on paper, and read which decisions belong before construction. The things that have to be decided before the permit are more numerous than most people think, and the list includes the windows, the stairs, and the location of every plumbing fixture.

Design to a budget, not the other way around

The process that works is backwards from how most people do it. Set the all-in number first, including the lot, servicing, permits, HST and a contingency. Take the build portion of that and have the designer work inside it, with the seven decisions above as the levers. Then draw. A plan that comes in at the number on the first try is not luck; it is a designer who priced as they drew.

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