Haven do negative equity trade-up mortgages, so they might. They seem very rare and I suspect that your salary would have to very high. If your salary is that high, maybe you could defer the extension and save up the difference?
Make sure that you are not entitled to a tracker on expiry of the fixed rate, just in case a new mortgage would cause you to lose it.
joint net income is about €6.1k/mth (only recently increased from about €5k) - mortgage after TRS is about €2.7k with no other debt/loans. Childcare €1k and the rest 'usual' bills - have only just got back to a position where we can put a little aside as savings. the €40k equity for extending would be our current savings. baby no 2 due early next year, hence the want to extend.
Pretty sure there's no tracker - we were'nt offered one at the outset and this is our second fixed rate, i believe when we re-fixed Haven had said we would have reverted to their SVR.
Brendan, many thanks for the input. i think for now we may just have to look at a lower cost solution for reconfiguring the current house to make it more livable.