Home › Additional dwelling units
Additional dwelling units and coach houses in Ottawa
ADU, ARU, SDU, coach house, in-law suite, basement apartment: homeowners hear all of these and fairly assume they are different things.
- Liability insurance and WSIB coverage
- 5-year warranty on construction
- 2-year warranty on painting
- Free in-home consultation
Additional dwelling units
What the terms mean
ADU, ARU, SDU, coach house, in-law suite, basement apartment: homeowners hear all of these and fairly assume they are different things. Mostly they are different names for the same idea, a second, self-contained home on your property. This page sorts out the terms, explains how the Ontario and City of Ottawa rules fit together, and covers the one type that sits outside the house: the coach house.
- ARU (additional residential unit) is the Province's term. Ontario's Planning Act uses it for the extra units it allows on most residential lots.
- ADU (additional dwelling unit) is the term the City of Ottawa uses now, for an extra unit inside the house or in a building behind it.
- SDU (secondary dwelling unit) is the City's older term for the same thing, and you will still see it everywhere. Our secondary dwelling units page covers units inside the house: basement apartments and in-law suites.
- A coach house is an additional dwelling unit in a separate, detached building on the same lot, usually in the back yard.
- In-law suite and basement apartment are everyday names, not legal categories. What matters is whether the unit is self-contained and legal.
“Indiana and his crew renovated a bathroom for me and the process was seamless, respectful and completed with extreme care and quality. I consider Carleton Prime the best construction company in Ottawa. I will use them for all future projects. Very pleased, would highly recommend.”
How many units a lot can have
Ontario's Planning Act lets most serviced urban residential lots have up to three units: all three in the main house, or two in the house and one in a separate building such as a coach house. The City of Ottawa's pages describe the same limit: up to two additional dwelling units, either two apartments in the house or one apartment and one coach house, for a total of three homes on the lot.
Lots on a private well or septic system are different. The City limits them to one additional unit or one coach house, and the septic system has to be able to handle it.
Ottawa's new Zoning By-law (2026-50) is now mostly in force, with some parts still under appeal. Until those appeals are settled, the City applies the more restrictive of the old and new by-laws. The new by-law would allow more units on some lots in neighbourhood zones, but the number of units allowed there is one of the rules under appeal, so for now the three-unit limit above is the one to plan around. That is why we check the current rules for your lot before anything is designed.
Coach houses
A coach house is a small, complete home in your back yard, with its own kitchen, bathroom and entrance. Families build them for a parent who wants independence close by, for a grown child, or for rental income. Carleton Prime builds coach houses as new buildings, from the foundation up, on city and rural lots.
Zoning limits how big and how tall a coach house can be, how far it sits from the lot lines, and where it can go on the lot, and the rules for urban and rural lots are different. In the urban area, the City requires the coach house to get its water and sewer through the main home's connections, which shapes where on the lot it can sensibly go. A building permit is required in every case.
Inside the house or out back?
A unit inside the house usually costs less, because the foundation, the roof and the services are already there. A coach house gives both households more privacy and leaves the main house as it is, but it is a new building from the footings up. Which one suits you depends on your lot, your house and what the unit is for. We look at both during the free consultation and give you a straight answer.
How we build them
Whether it is a basement suite or a coach house, the work follows our usual five steps, and we work out which permits it needs and apply for them on your behalf. Where a unit needs drawings from a designer or engineer, we build from those drawings. Licensed trades that we coordinate do the plumbing, electrical and HVAC, and our own team does the framing, drywall, flooring, tile, trim and painting. For landlords, our rental property page covers adding a unit as an investment.
Whole job or one part
The whole project, or any part of it
Carleton Prime can take on the whole project, turnkey, or just the part you need: one phase, or one trade such as the framing, drywall, tile, flooring or painting. Builders, general contractors, architects, designers and engineers can also bring us in as a subcontractor and an extra, experienced member of the team.
Frequently asked questions
Is an ADU the same as a secondary dwelling unit?
Yes, in practice. Additional dwelling unit is the City of Ottawa's current term, and secondary dwelling unit is its older term for the same thing. The Province calls them additional residential units.
How many units can I have on my lot in Ottawa?
On most serviced residential lots, up to three in total: your home plus two additional units, either both in the house or one in the house and one coach house. Lots on a well or septic system are limited to one additional unit or coach house.
Do you build coach houses?
Yes. A coach house is a separate, detached home on your lot, usually in the back yard, with its own kitchen, bathroom and entrance. We build them as new buildings, from the zoning check and permit through to the final walkthrough.
Does a coach house need a building permit?
Yes. The City of Ottawa requires a building permit for a coach house in every case, and the design has to meet zoning.
How much does a coach house or basement unit cost?
We quote each one on its own scope. For a coach house, the size, the foundation, how water and sewer reach it, and the finishes drive the price. For a basement unit, it is mostly fire separation, plumbing, the entrance and windows.
Can I build a coach house on a rural lot with a well and septic?
Possibly. Rural lots have their own zoning rules, and the septic system has to be able to handle the extra unit, so we look at both early.
Have the rules changed recently?
Yes. Ottawa's new Zoning By-law 2026-50 is now mostly in force, but some parts, including how many units neighbourhood lots can have, are still under appeal. Until those are settled, the more restrictive of the old and new rules applies. We check the rules for your lot before anything is designed.
Do you build basement apartments too?
Yes. Units inside the house are covered on our secondary dwelling units page.
Related services
Related service
Secondary dwelling units
Legal basement apartments and in-law suites, built to the Building Code.
Learn moreRelated service
Custom homes
New homes on city and rural lots, framed and finished by one company.
Learn more
Related service
Rental and multi-unit
Unit turnovers, new suites and upgrades planned around tenants.
Learn moreSources
The permit, Building Code and bylaw points on this page come from these authorities. Rules change, so check the current version before relying on any of it.
- Government of Ontario: Zoning bylaws (Citizen's guide to land use planning), additional residential units
- City of Ottawa: Adding a coach house (additional dwelling units in an accessory structure)
- City of Ottawa: Adding an apartment (additional dwelling units)
- City of Ottawa: New Zoning By-law (2026-50)
- City of Ottawa: Update on the status of appeals to the new Zoning By-law (memo, 7 Jul 2026)
- Government of Ontario: Ontario's Building Code
Ottawa renovation and construction
Start your project with Carleton Prime
Every project starts with a free in-home consultation. Tell us about your home and what you want to do.