We've been having this exact conversation here tonight and as much as I was keen to stay put, the numbers say move if we want more space, and that's ignoring the multitude of other ways in which the house is failing us. Not sure I want to live through a loft conversion either.
So to continue the eg, say the previous place is 600k with 2% agent fees + 2k of solicitor fees + 1k moving costs and we're at £40k vs >£70k.
Don't improve, move.
Edit that's not right, as I'm not inc. mortgage porting costs and buyers solicitors costs so what £45k?