If we get a timber-framed single-storey extension, can we later (in a few years) add another storey on top?
Can you - yes - with the application of sufficient money, anything is possible.

Will it be cost-effective - probably not. Building, and then destroying a roof is a very expensive proposition. Removing roof framing and replacing it with an additional floor, likewise.

If you wanted to do this in a way that might make fiscal sense, have the frame designed as a two-story frame, but leave the 2nd floor out at first. It will cost somewhat more than a single-story, but it can actually be built out later without costing an absurd amount of money.