C/AS1 - Large workshops in residential unit

In C/AS1, at what size does a garage (2 cars, 3 cars, 4 cars…) become a “workshop” requiring fire separation from the sleeping spaces?

C/AS1 does not require fire separation of the garage of a dwelling unit but it doesn’t mention relative size.

Intuitively, a garage for 2 or 3 cars is a reasonable garage for an average house of 2 or 3 levels. But what if the dwelling is the same size or smaller than the garage?

For example, on a lifestyle block the owner wants to built a detached hobby garage/workshop for 3 to 4 cars and create a small dwelling unit above part of the double height building.

My thoughts are:
If the garage is for the exclusive use of the occupier then it does not require fire separation (ie C/AS1). OK for 1 maybe 2 cars, but what if it’s a large, double height and the occupant is a single person with a car collection?
If not, then it is C/AS2.

What does the fire community think?

it is not an issue if the garage is under SH (use and risk group) in NZ. If it is directly associated with the household and that is SH the garage is SH…if for commercial gain than WB possibly…

I’d say single residential freehold site, zoned residential then a detached shed regardless of size is SH. I think there’s no established middle ground or established rule that defines a size that would trips WB. I would argue regardless of the owner being into miniature railway or real size trains the building remains SH. And for what it worth, my view has always been the same when it comes to buildings for home owner occupations where they don’t have staff.

What about the customers?

The home owner is in full control of the customer. Its there home property, they say who comes in or not. Otherwise the government would be sending squads around the country to turf out (for example) all the home based child care business with 2-5 kids. These for example do not have any commercial building or commercial business facilities and I believe that’s correct. Not that I have any council decision making powers any more.

If customers are visiting the premises, it sounds like a home-based business. Auckland Council have a Practice Note AC2201 for home-based businesses, my reading is that it advocates a mixed-use design (e.g. pulling from both C/AS1 and C/AS2 to suit). This seems a sensible approach that gives us some leeway to tailor the design to the intended use, supported by a statement from the owner which accompanies the consent application.

The original question seems to be talking about a private use (not a home-based business). I would agree that C/AS1 doesn’t require fire separation to the garage, but perhaps this is a good opportunity to talk to the client and ask whether they want any protection above the minimum level required.

I did a recent project where we recommended additional fixings to the linings of the wall between the garage/workshop and the living space, essentially fixing the normal plasterboard as a GIB GBTL30c system to provide some level of protection at minimal additional cost. Smoke stopping (near ceiling level?) and/or interconnected alarm could be useful on a case-by-case basis.