What coach house building involves
A coach house takes its name from the original carriage houses built beside older Ottawa homes to shelter a horse and carriage, with living quarters for staff above. The modern version does the same job in principle: a detached Additional Dwelling Unit positioned above or beside a garage or parking area, so the unit’s footprint overlaps with space you were already using for parking instead of taking a bite out of open yard.
On a lot where the rear yard is narrow, sloped, or already committed to a pool, deck, or mature landscaping, a coach house can be the only detached ADU option that actually fits – because it builds up over the garage footprint rather than out across the yard. It is also a natural fit for a property that already has a detached garage in reasonable structural condition, since the garage level can sometimes be retained or reinforced rather than demolished outright.
Putting a living unit above a garage means the garage’s roof structure has to become a proper second-storey floor, which is a different engineering problem than a garage roof was ever designed to solve. That usually means new floor joists rated for residential occupancy loads, upgraded wall framing to carry that load down to the foundation, and a stair providing direct access to the unit that does not run through the garage itself when the garage remains in active use below. We assess this at the design stage, before committing to whether an existing garage is a renovation candidate or whether a full rebuild is the more cost-effective path.
A coach house typically gets its own exterior stair and entrance, separate from both the garage door and the main house’s front door. On lots with rear lane access – more common in some of Ottawa’s older neighbourhoods – that lane can sometimes double as a private approach to the coach house, keeping its comings and goings separate from the main house’s front yard entirely.