Ask a few local estate agents what the change in value would be. If it is much less than the cost + hassle, you might decide not to bother.
I think you need to figure out where you want to be in x years,
see if both options (sell up or extend and stay) would work to get there,
and if so then cost both options end-to-end, ruthlessly including all costs
and bearing in mind any value you might add to the house,
and then decide.
If only one option works then you need to see have you got other options.
If you have only one option that works then do that.