Garden suite / ADU construction cost calculator

Build a bottom-up cost plan for a backyard suite. This is a price planner, not zoning or permit approval.

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Calculation breakdown

What this result includes—and excludes

The numbers are editable planning scenarios and are not professional advice, quotes or eligibility decisions. No entered information is submitted to a server by this calculator.

How the calculation works

Base = area × user-selected construction rate. Pre-contingency = base + sitework + utilities + plans + permits. Total = pre-contingency × (1 + contingency %) × (1 + tax allowance %). No jurisdiction-specific buildability approval or verified construction price is inferred. The optional tax allowance is applied to the entire post-contingency subtotal; that is a simplification and does not decide which costs are taxable. Request a line-item tax treatment from your suppliers.

Understand the budget before you apply for a building permit

A detached garden suite (sometimes called a backyard accessory dwelling unit, coach house or laneway home) can involve construction and approval work separate from the existing home. This is a cost-planning model only. A municipality may have lot-specific limits on siting, setbacks, lot coverage, access, parking, trees, building height, servicing and permit requirements. Provincial housing policy does not mean that every particular property or design is automatically approved.

Before relying on a construction price, ask a designer or builder to confirm whether the square-foot quote includes the foundation, design, servicing, site grading, municipal fees and taxes. If one of those items is already included in the unit rate, enter 0 for the same separately listed allowance to prevent counting it twice. A utility company may also require work outside your property that the initial quotation does not include.

Worked example using the editable starting values

A 650 sq. ft. garden suite at an illustrative C$350 per sq. ft. has a C$227,500 construction allowance. The sample adds C$25,000 in foundation/sitework, C$22,000 in utility connections, C$18,000 for design and engineering, and C$8,000 in permits/fees. Pre-contingency cost is C$300,500; a 15% reserve adds C$45,075, giving a modelled total of C$345,575 before taxes not entered. None of those illustrative line items is an official local fee or a validated Canadian average.

Five checks before treating an ADU budget as realistic

Check the property's rules. Confirm the address, adopted municipal bylaw, permitted unit type, setback/height restrictions, lot configuration, servicing constraints and applicable building code with the municipality. Do not infer permission solely from a province-wide policy overview.
Request an itemized contractor scope. Find out what the cost per square foot includes: foundation, framing, roof, envelope, kitchen/bath, appliances, heating/ventilation and connection work.
Confirm servicing. Review water, sewer, stormwater, electrical capacity, access, utility upgrades and any off-site connection requirements. These vary by lot and utility provider.
Verify soft costs and municipal charges. Ask about surveys, design, engineering, energy compliance, development-related charges and permits; include each cost once.
Check taxes and reserve funding. The default 0% tax is a blank user assumption, not a tax exemption. Applicable GST/HST treatment can depend on the nature of the work and supplier; obtain professional advice as required.

The ±15% construction-rate sensitivity shown in the results varies only the entered construction unit rate; the other costs and percentage allowances stay fixed. It is a planning stress test, not a prediction or a local pricing dataset.

Sources and further checking

Source and method review: October 11, 2026. Government references explain tax, zoning or regulatory context; none validates the sample price inputs. Check current local rules, tax treatment and actual contractor quotations before making decisions.