Housing/remortgage options

[MoneyFunny]

We have our house £150k mortgage worth £220k/230k now.
We want to stay put, and there is just enough space to do so, nice area, near school, small third bedroom for 2nd DC when they hopefully come along. Ideally we want to extend (shouldn’t be major issues, neighbour has extension, relative is a planning officer and another is an engineer so we have a good idea of options) but quotes are looking at £100k for single storey to add dining/utility/WC shower and extra lounge).
Not enough equity currently to borrow all of that and want a second DC so don’t want to get a loan as well as remortgage.
My question is - we remortgage and get say, £50k, where do we put the money until we are ready?
Normal account? ISA? S and S?
Or do we not remortgage at all!

No point remortgaging for the £50k until you know where the other £50k is coming from and you've got planning permission

Really? I wondered if it was better to have it ready so we can pay for costly starter fees such as planning/architect and engineer drawings etc?

How much is that going to cost though roughly? Could you afford to save that instead
But if you haven't got the money to do the extension yet, there's no point in taking a massive chunk out of your equity if you don't know how you're going to get the rest of the money to pay for the extension in the meantime. It's going to increase your mortgage payments. Surely you're better off saving what you can to make up the other £50k while you're paying the lower mortgage payment