One other point to add to Sara's extremely helpful reply. If you think that you might be able to reduce the amount of your mortgage, stick with a variable rate, on which there are no penalties for part redemption. Then, if it turns out that late in the day that you need less, you can perhaps avoid the hassle of having to get out a fresh loan offer, and simply return a chunk of the loan cheque straight to your lender.
Similarly for self-build houses, ALWAYS apply for as much as you can get, and leave it on a variable rate. You can always just not take the final instalment if it turns out you don't need it. As a practising solicitor, I hate the situation of having to do a second mortgage for an extra €10k needed urgently due to budget over-runs.
It's a client relations disaster area: No matter how little I charge, it will seem big relative to the sum being obtained.
Nett result: unhappy client, unhappy solicitor.