Home additions in Ajax

The zone on your lot sets the room you have. These are the house standards in By-law 95-2003, Table 2, as of October 2026:

Note: these are the printed by-law minimums and maximums; site-specific exceptions in section 7 can change them, and the 90% rule below shortens the setbacks.

Coverage is where R1-A and R1-B lots run out first. Ajax measures it as the ground floor area of all buildings over the lot area, and the definition leaves out porches, unenclosed verandas and platforms. Your deck doesn't count against it. A sunroom with walls does. On a 20.0 m by 33.5 m R1-A lot, the minimum size, 30% is 201 m² of footprint for house, garage and shed together (our arithmetic).

The rear yard is measured from the lot line to the nearest wall of the house. Under O. Reg. 545/06, a 7.5 m minimum now reads as 6.75 m and 9.0 m as 8.1 m, without a variance. On a 7.5 m yard that is 0.75 m more depth across the back. The Greenbelt is excluded, as are the areas Ontario prescribes near valleys, wetlands, the Lake Ontario shore and railways, so a lot backing onto a creek valley may not get it. The setback in force when the permit is issued is the one you keep.

Ajax's Committee of Adjustment meets monthly, on Wednesdays, and its own page names the requests it sees most: side-yard steps down to a below-grade entrance, and bigger decks. Those sit in a cheaper 2025 fee line of $845.90, alongside accessory buildings and driveway widenings. An addition that breaks coverage or the rear yard pays the house rate. Build first and ask later and the fee doubles. Pre-consultation with staff costs $687.30 and the form asks for it. After the hearing there's a 20-day appeal period, and Ajax won't issue the building permit until it ends.

If your lot is regulated, the conservation authority adds its own review fee to the variance: TRCA charges $695 for a minor residential one, CLOCA $930 (CLOCA 2026 fees).

Most of Ajax drains to Duffins Creek or Carruthers Creek and answers to TRCA. The east end is CLOCA's: its regulation maps include sheets titled Lakeshore Ajax and Northeast Ajax. The two price the work differently. CLOCA's Minor B permit, $535 in 2026, covers minor additions with no technical report. TRCA puts additions at Standard ($1,045) or Major ($1,490 and up), and an addition over half the original ground floor at Complex, from $2,840. Neither name lasts long: Ontario folds both Ajax authorities into a single Central Lake Ontario Regional Conservation Authority as of 1 February 2027.

Greenwood, a Toronto house about 32 km west of Ajax, is the closest rear extension we've finished. It ran 10 months from permit to handover, with the design and the build handled by the same team.