There are several types of std 2 up 2 down terrace houses – some basic floor plans would be of help. Friends of mine have looked inot similar options in the past, but once drawn have found that the upstairs layout becomes very inefficient due to the amount of corridor you need to gain access to each room. The position of the stairs usually governs what is and isn’t feasible. Also need to consider position of existing windows, ventilation etc.
As mentioned above, getting the waste out will be your biggest problem and may involve more intrusive work to the ground floor than you had bargained for. Doesn’t mean it can’t be done though. Water supply should be easier to achieve, but still needs to be thought through.
Is an attic conversion not do-able?