above garage addition

has anyone added a room above their garage recently? we want to add space to a house we are purchasing on the sunny coast (qld). One of the ideas we have is to build above the old attached garage. its 3.8 x 6.2 m on the floor plan. Not sure of the existing construction until friday when we have arranged building and pest inspection. i found something on a home reno page somewhere that suggested $3k per square metre which would come to $70k app. does that appear realistic? the planned space will be an extra bedroom so no plumbing, just walls, floor, windows and roof.

I'm surprised no one has responded.

This sounds like a question for an engineer. They'd probably want to know details of the existing walls and footings before being able to assess what kind of weight the garage could support.

I've never done what you're thinking. Though, like most other people, I chuck stuff through the access hole into the roof space for long-term storage. Eventually, I fitted foldable stairs, ran some extra bearers between the existing trusses and put down a floor, built some walls, and had a window fitted into the roof. There were no heavy items up there because everything had to be carried up the stairs.

You can’t load trusses up like that. It’s a big no-no

Where's the additional load on the trusses? Any extra static load seems to be taken by the new joists and flooring, unless the new joists were lain on top of the trusses horizontal members. But if it's only cardboard boxes I'd be happy

That house, which was rectangular in shape with a central passageway that was quite wide, lent itself to doing this. The existing trusses rested on the central passageway walls. The new joists I fitted ran from the external wall over both walls of the central passageway as well. I can't recall the distance between trusses, but I recall that you could easily step between one and the other when up there. How long is a step? I don't know. Maybe a couple of feet. So, the new joists would have halved that distance, which seemed a reasonable arrangement for the boards. And the walls I put in may have added to the strength of the existing trusses.

In the end, we never put much up there. It was too nice for junk. A few long-term things in suitcases, a couple of beanbags and it became a nice place to read as the sun came steaming in of a morning.