Three. On urban residential land in Ontario, the Planning Act, as changed by Bill 23 in November 2022, stops any town from prohibiting three units on a lot: three in the house, or two in the house and one in a backyard building. As of October 2026 several towns allow more, including Whitby and Ajax in Durham Region, Ontario, where four homes can share a lot, and Toronto, which allows four citywide and six in parts of the city.
Key takeaways
- Under section 35.1 of the Planning Act, a town can't prohibit 3 units on urban residential land, or require more than 1 parking space per added unit.
- Whitby allows up to 3 additional dwelling units, 4 homes in all, with a backyard unit of up to 75 m², per Zoning By-law 1784.
- Pickering caps a lot at 2 additional units, but its backyard unit can reach 150 m², per Zoning By-law 8149/24.
- A 2nd or 3rd unit in an existing house pays no development charges under section 2(3.2) of the Development Charges Act.
- Durham Region issued permits for 1,004 additional residential units in 2025, up from 999 in 2024, per the Region's 2025 building activity review.
What does Bill 23 let you build on your lot?
Bill 23 set a provincial minimum of three units on urban residential land, and towns can only add to it. "Urban residential land" means a lot inside a settlement area that's on municipal water and sewer. On that land, as of October 2026, section 35.1 of the Planning Act says a zoning by-law can't prohibit either layout:
- three units inside the house, or
- two units inside the house plus one in an ancillary building, such as a garden suite or a unit over a garage.
The same section says zoning can't demand more than one parking space per added unit, and can't set a minimum unit size. O. Reg. 299/19 adds that anyone can live in the units, related to you or not, and that the age of the house doesn't matter.
Two later regulations loosened things further. Since 20 November 2024, a lot with additional units can cover at least 45% of its area, faces no floor space index limit, and can't be made to keep more than 4 m between residential buildings on the lot (O. Reg. 462/24). Since 21 November 2025, urban residential setbacks are deemed to be 90% of the by-law figure, except near shorelines, wetlands, valleys and railways and in the Greenbelt, under O. Reg. 545/06. Site plan control doesn't apply to a lot of 10 units or fewer unless it's in a prescribed area.
How many units does each Durham town allow?
Durham's five lakeshore towns split two ways: Whitby and Ajax allow four homes on a lot, while Oshawa, Pickering and Clarington stop at three. As of October 2026, here's what each zoning by-law or town page allows on a typical serviced lot:
| Town | Homes on one lot | Backyard unit limits | Source |
|---|---|---|---|
| Whitby | 4: up to 3 ADUs, either 2 in the house plus 1 accessory, or 3 in the house; 1 ADU on Moraine and Greenbelt lots | 75 m² GFA, 4.5 m high, 1.2 m interior side and rear setbacks | Whitby By-law 1784, s.4 |
| Oshawa | 3, with at most 1 in an accessory building | All accessory buildings capped by 8% of lot area, 50% of house coverage or 60 m² of ground floor (90 m² in R1-G and R1-H); 4.5 m high; 0.6 m side and rear | Oshawa accessory apartments page |
| Ajax | 4: up to 3 attached ADUs, or 2 attached plus 1 detached | At least 3.0 m from other buildings; ground floor smaller than the house; 45% lot coverage with an ADU | Ajax By-law 95-2003, s.4.4 |
| Pickering | 3: 2 ADUs in the house, or 1 in the house plus 1 accessory; 1 on the Moraine | Lesser of 150 m² or 50% of the house's GFA; 4.5 m high on lots up to 2,000 m²; 1.2 m side and rear | Pickering By-law 8149/24, s.4.3.1 |
| Clarington | 3 in Courtice, Bowmanville, Newcastle and Orono: 1 in the house plus 1 accessory; 1 on Moraine and Greenbelt land | 1.8 m rear and interior side setbacks; 5.25 m high in urban zones; smaller than the house | Clarington ADU page |
Note: these are general rules for serviced lots as of October 2026; hazard lands and site-specific zoning can cut them back.
The backyard unit's size matters as much as the count: Pickering's 150 m² ceiling is double Whitby's 75 m². Where a town's page describes fewer layouts than the Planning Act guarantees, as Clarington's does, the provincial layouts still apply on urban residential land; ask the planning department.
Many north Durham lots are on wells and septic, outside the provincial three-unit rule. Scugog allows up to two ADUs in settlement areas such as Port Perry, per its ADU page. Uxbridge permits a two-unit house wherever a single, semi or row house is allowed, per its zoning by-law. Brock allows two units in the house, or one in the house and one in an accessory building, per its zoning by-law.
How many units do GTA towns allow?
Across the rest of the GTA, the count runs from three to six. As of October 2026, each row was checked against the by-law or town page it links to:
| Town | Homes on one lot | Backyard unit limits | Source |
|---|---|---|---|
| Toronto | 4 as of right citywide in R, RD, RS, RM and RT zones; 6 in Toronto and East York and Ward 23 (sixplex rules) | Garden suite up to 120 m² GFA (60 m² if one storey), 2 storeys, smaller than the house | Toronto multiplex rules |
| Mississauga | 3: 2 attached ARUs, or 1 attached plus 1 garden suite; new fourplexes in RL, RS and R16 zones | 55 m² on lots up to 550 m², otherwise 10% of lot area up to 100 m² | Mississauga zoning, Part 4 |
| Brampton | 3, including the house: the house plus 2 attached units, or 1 attached unit and 1 garden suite | Units must be registered with the City | Brampton second units page |
| Caledon | 4 as of right town-wide | Not covered in our source; the Town runs an ARU registration program | Caledon housing page |
| Markham | 3: up to 2 ARUs in the house, or 1 in the house plus 1 in an accessory building | Garden suites only through a temporary use by-law | Markham By-law 2024-19 |
| Richmond Hill | 4: up to 3 ARUs plus the house | At most 2 of the ARUs in an accessory residential building | Richmond Hill ARU page |
| Newmarket | 3: up to 2 ARUs | Lesser of 10% of lot area or 75 m² one storey; 100 m² at two storeys | Newmarket By-law 2026-10 |
| Aurora | 3, including the house | Not covered in our source | Aurora registration By-law 6660-24 |
| Whitchurch-Stouffville | 4 on serviced lots with 15 m frontage and 555 m² area | 56 m² on lots under 500 m², rising to 233 m² on lots of 1,500 m² or more | Stouffville zoning, Section 3 |
| Georgina | 3: up to 2 ARUs | Detached unit 3 m to 50 m from the house | Georgina By-law 500, s.5 |
| Oakville | 3: 2 attached, or 1 attached plus 1 detached | Not covered in our source | Oakville By-law 2014-014 |
| Burlington | 4: up to 3 ARUs, up to 2 detached in one accessory building | 80 m² GFA; 7 m with a peaked roof, 6 m flat | Burlington By-law 09-2026, Part 5 |
| Milton | 4: up to 3 ADUs, 1 in an accessory building, on municipal water and wastewater | Above-grade units up to 85 m² each; a basement unit can use the whole basement | Milton's zoning by-law 016-2014 page |
| Halton Hills | 3: 2 ARUs in total | Lesser of 90 m² or 40% of the house; 6.5 m high | Halton Hills By-law 2010-0050 |
Note: these are summaries of each town's general rule for a serviced lot as of October 2026, not a check of your property; Oak Ridges Moraine, Greenbelt and floodplain lots usually get fewer units or none.
Is a third unit harder to build than a second?
It can be, because the Building Code treats the house differently once it holds three units. The 2024 Building Code defines a secondary suite as a unit in a building that holds only one other dwelling unit (Div. A 1.4.1.2.). Several easier rules, such as the Type X drywall alternative to a rated fire separation, apply to a "house with a secondary suite". Add a third unit inside the same house and those shortcuts may no longer fit, so talk to the building department before you design around them.
A third unit in a backyard building avoids that problem, since the house still holds two. Our requirements guide lists the Code rules for a two-unit house, and the cost guide prices a basement unit.
Do additional units pay development charges?
No, within the provincial limits. As of October 2026, a second or third unit in an existing house, or one unit in an ancillary building, pays no development charges under section 2(3.2) of the Development Charges Act, and section 2(3.3) does the same for new houses.
Whitby notes one exception on its ADU page: education development charges still apply to a detached ADU. Whitby's Housing Accelerator Fund incentive reimburses them, along with the permit and entrance fees, for complete applications made by 31 December 2026. See our Whitby ADU incentive guide, and basement permits in Durham Region for each town's permit fees.
What still limits what you can build?
Setbacks, height and the land itself still limit you. Oak Ridges Moraine lots in Whitby, Pickering and Clarington drop to one additional unit, and so do Greenbelt lots in Whitby and Clarington. Land regulated by a conservation authority needs its permit too: Whitby and Oshawa sit under Central Lake Ontario Conservation (CLOCA), and Ajax and Pickering under both CLOCA and the Toronto and Region Conservation Authority. Ajax bars ADUs on lands its conservation authority marks as hazardous, per By-law 95-2003.
If your plan misses a setback or a height limit, you need a minor variance from the Committee of Adjustment. In 2026 the residential fee is $1,526 in Whitby, $856 per lot in Oshawa, $2,500 in Pickering from 1 July 2026, and $2,280 in Clarington. Ajax charged $1,638.90 for a single residential variance in its 2025 user fees; we didn't find a 2026 schedule.
How we help you decide what to add
We're Jarvis Contracting, a family design-build firm based in Ajax. James Jarvis runs construction and his wife, Ceara Jarvis, runs design through her studio Juliette Interiors. We draw permit drawings in-house and build basements and additional units, so we can lay out a basement suite and a garden suite on one lot and show you what each needs before you commit. The town reviews the drawings and issues the permit. See garden suites, basement renovations and how our design-build process runs.
Questions
How many units can I build on my lot in Whitby?
As of October 2026, Whitby allows up to three additional dwelling units on a serviced residential lot, four homes in all. They can be two in the house plus one in an accessory building, or three in the house. A backyard unit is capped at 75 m² of floor area and 4.5 m in height, and Moraine or Greenbelt lots get one additional unit only.
Does Bill 23 apply to rural lots in Ontario?
Bill 23's three-unit rule applies only to urban residential land, meaning a lot inside a settlement area on municipal water and sewer. Rural lots on wells or septic depend on the town's own zoning. In north Durham, Scugog allows up to two additional units in settlement areas such as Port Perry, and Uxbridge allows a two-unit house.
Can I have a basement apartment and a garden suite on the same lot in Ontario?
A basement apartment and a garden suite can share a lot. On urban residential land the Planning Act stops a town from prohibiting two units in the house plus one in a backyard building. Each town still sets the garden suite's size, height and setbacks, from 75 m² in Whitby to 150 m² in Pickering.
Do I need extra parking for an additional unit in Ontario?
A town can require at most one parking space for each additional unit under section 35.1 of the Planning Act, and some require none. Whitby waives parking for units in a Protected Major Transit Station Area, and Burlington asks for none for the first additional unit.
Can I rent an additional unit to someone who isn't family?
An additional unit in Ontario can be rented to anyone. Ontario Regulation 299/19 lets anyone live in an additional residential unit, related to the owner or not, and it applies however old the house is.
Sources
- Planning Act ontario.ca
- Zoning By-law 1784 media-003-ca.cdn.govstack.com
- Zoning By-law 8149/24 corporate.pickering.ca
- Development Charges Act ontario.ca
- 2025 building activity review media-003-ca.cdn.govstack.com
- O. Reg. 299/19 ontario.ca
- O. Reg. 545/06 ontario.ca
- Oshawa accessory apartments page oshawa.ca
- Ajax By-law 95-2003, s.4.4 ajax.ca
- Clarington ADU page clarington.net
- ADU page scugog.ca
- zoning by-law uxbridge.ca
- zoning by-law media-002-ca.cdn.govstack.com
- sixplex rules toronto.ca
- Toronto multiplex rules toronto.ca
- Mississauga zoning, Part 4 www7.mississauga.ca
- Brampton second units page brampton.ca
- Caledon housing page caledon.ca
- Markham By-law 2024-19 markham.ca
- Richmond Hill ARU page richmondhill.ca
- Newmarket By-law 2026-10 newmarket.ca
- Aurora registration By-law 6660-24 media-002-ca.cdn.govstack.com
- Stouffville zoning, Section 3 townofws.ca
- Georgina By-law 500, s.5 georgina.ca
- Oakville By-law 2014-014 oakville.ca
- Burlington By-law 09-2026, Part 5 burlington.ca
- Milton's zoning by-law 016-2014 page milton.ca
- Halton Hills By-law 2010-0050 haltonhills.ca
- 2024 Building Code publications.gov.on.ca
- ADU page whitby.ca
- Whitby media-003-ca.cdn.govstack.com
- Oshawa oshawa.ca
- Pickering corporate.pickering.ca
- Clarington weblink.clarington.net
- 2025 user fees ajax.ca