We probably can't afford to get 2 storeys built at the moment but don't want to close off that option for future years (when we may want an extra bedroom). We are thinking of a timber-framed extension for environmental reasons: trying to avoid too much concrete. Thanks.
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.