ADU Floor Plans: What Fits in the Space You Have

HouseLyft does not sell plan sets, and nothing here is permit-ready.

Most people start with a plan they like and work backwards. That’s the expensive order. The plan that matters is the one that fits inside the envelope your bylaw already permits — and calculating that envelope takes about twenty minutes with your zoning bylaw open.

This guide is that calculation, then what fits inside the result.

(Our companion guide on ADU design covers design principles and climate constraints. This one is the practical sizing sequence.)

How to calculate your buildable envelope

Six steps. You need your zoning bylaw and, ideally, a survey.

Step 1 — Get your zone designation and site area.

Use your municipality’s mapping tool. Edmonton directs homeowners to maps.edmonton.ca and also publishes open data queryable by address. Calgary provides an interactive address map with a land use district lookup. Most Ontario and BC municipalities publish zoning maps online.

Write down: your zone, your site area, and your lot dimensions.

Step 2 — Find the six numbers in your bylaw.

Edmonton’s backyard housing provisions are the clearest published example: 1.2 m minimum setback (0.6 m at the rear abutting an alley where no garage door faces it), 20% site coverage, 6.8 m height — dropping to 4.3 m where the rear lot line abuts a residential site limited to 12.0 m or less — 60.0 m² maximum second-storey floor area (70.0 m² with Inclusive Design), and 3.0 m separation from other principal buildings.

Toronto’s garden suite provisions set a 60 m² total floor area, a rear setback of the greater of 1.5 m or half the building height on lots deeper than 45 m, and a 45-degree angular plane from 4.0 m measured from the required rear and side setbacks.

Step 3 — Draw the lot and apply the setbacks.

Graph paper or any drawing app. Outline your lot to scale, then draw the setback lines in. The shape left in the middle is your starting footprint.

Step 4 — Subtract parking and amenity space.

This is the step people skip, and it’s often what kills a plan.

In Calgary, a secondary suite requires one parking stall for the tenant in addition to the property’s own requirement, with published minimum dimensions — 2.5 m × 5.9 m free-standing, 2.85 m × 5.9 m abutting a structure on one side, 3.0 m × 5.9 m abutting on both sides. Street parking doesn’t count, and tandem stalls behind other required stalls don’t count.

Calgary also requires amenity space of at least 7.5 m² with no side under 1.5 m, shown on the site plan.

Draw both onto your lot. What remains is genuinely buildable.

Step 5 — Apply coverage and height.

Multiply your site area by the coverage percentage to get maximum building footprint across all structures — then subtract what your existing house and garage already occupy. That’s frequently the binding constraint on a rear-yard build, and it surprises people.

Then apply the height limit, and check whether any reduced limit applies to your rear lot line.

Step 6 — Apply the angular plane and floor-area cap.

Where an angular plane applies, sketch it in section — it slopes the envelope inward above the trigger height and determines whether a full second storey is achievable.

Then check whether your floor-area cap is a total or a second-storey limit. Edmonton’s 60 m² caps the upper floor only; Toronto’s 60 m² is a total. Same number, very different building.

What you now have is a three-dimensional box. Every plan you consider has to fit inside it.

What fits at each size band

Described, not drawn, and none of it permit-ready.

30–40 m² — studio. One room doing everything plus a separate bathroom. Works where the envelope is genuinely tight. Check local demand first: in many Canadian markets a studio rents disproportionately below a one-bedroom, so the saving may not be worth it.

40–55 m² — one-bedroom. The best-value configuration for most Canadian lots. Open living/kitchen/dining along the best-lit wall, a separate bedroom, a bathroom. Largest tenant pool, least area wasted on circulation.

55–70 m² — large one-bedroom or tight two-bedroom. At the lower end, two bedrooms means both are small and the living space compresses. Research whether your market actually pays a premium for the second bedroom before committing — often it doesn’t pay the cost difference.

70 m²+ — comfortable two-bedroom. In Edmonton, this is what the Inclusive Design route buys you: 70 m² rather than 60 m² on the second storey.

A recurring rule: in a small plan, every square metre in a corridor is one not in a room. Plans that work put circulation through rooms rather than beside them.

One storey vs two

One storey is simpler, cheaper per square metre, has no stair consuming floor area, and works where height is restricted. On a lot where the rear setback height limit drops — Edmonton’s 4.3 m provision, for instance — it may be your only option.

Two storeys get more floor area from a smaller footprint, which matters where site coverage rather than floor area is your binding constraint. That’s common on lots that already have a house and a garage.

Where two storeys is permitted depends on:

• The general height limit (Edmonton 6.8 m, Vancouver R1-1 infill two storeys up to 8.5 m)
• Any reduced height limit at your rear lot line
• Any angular plane, which can make the upper floor unusably narrow even where the height is technically available
• Second-storey specific caps — Edmonton limits any second-storey building wall to 15.0 m in length

Check the reduced height limit before assuming two storeys. It’s the most common reason an over-garage design doesn’t work.

Where the plumbing wall goes

An unglamorous constraint that dictates more layouts than aesthetics do.

The principle: group the kitchen and bathroom on a shared wall, positioned as close as practical to where water and sanitary sewer arrive at the building. Every metre of horizontal drainage run adds cost, requires fall, and eats ceiling space.

In practice:

• Find where services will enter — usually the elevation facing the main house or the alley
• Put the wet wall on or near that side
• Stack bathroom above kitchen in a two-storey plan, or put them back-to-back on one level
• Keep the living space and bedroom on the better-lit elevations

The cost consequence is real: a plan with the bathroom at the far end from the services can add meaningfully to plumbing cost and complicate the floor structure. It’s invisible on a drawing and obvious on an invoice.

And on a rear-yard building, remember the servicing run itself — trenching water, sewer and electrical the depth of the lot — is frequently the largest single line item in the whole project.

Accessibility

Worth designing in, for three reasons.

It can buy floor area. Edmonton allows a 70 m² second storey rather than 60 m² where the building meets the bylaw’s Inclusive Design requirements under Section 5.50. That extra 10 m² is often what makes two bedrooms work.

It can attract funding. Calgary’s Secondary Suite Incentive offers up to an additional $7,500 for accessible spaces serving common mobility issues — on top of the base safety-element funding.

It earns points at larger scale. For projects of five or more units, CMHC’s MLI Select awards points across affordability, energy efficiency and accessibility, converting them into higher loan-to-value and longer amortisation across three tiers. Accessibility is one of the three criteria.

All of these are drawing-stage decisions. Level entry, wider doorways, blocking for future grab bars, a curbless shower and adequate turning space have to be designed in — they can’t be retrofitted economically. Decide before you draw.

And a practical point beyond the incentives: an accessible unit serves an ageing parent now and a wider tenant pool later. That optionality is worth something regardless of programmes.

Getting a plan drawn

Check the free resources first. Toronto publishes “Made in Toronto” pre-approved plans for garden and laneway suites, intended to make building an additional unit faster and less expensive. CMHC’s Housing Design Catalogue is a free national resource including laneway and garden suite designs. If either suits your envelope, it can cut design cost and review time materially.

When you do brief a designer, bring: your zone and site area, the six dimensional numbers, a survey, where services enter, any protected trees, your target configuration and the rent evidence behind it, and whether you’re targeting an accessibility or energy incentive.

And be clear about what you’re buying. Concept sketches, permit drawings and construction drawings are different deliverables at different prices. Ask which you’re getting.

To be explicit: HouseLyft does not sell plan sets and does not provide permit-ready drawings. We establish what your property can support, which is the input a designer needs. Our planning and permits page covers the approvals stage and how it works covers the sequence.

Get your envelope calculated properly

The envelope is the input to every other decision, and getting it wrong means paying for drawings that don’t survive review.

HouseLyft’s free property assessment establishes your buildable envelope, your servicing constraints and what configurations are realistic — before you spend anything on design. Request your free report.