Once ever in years of dealing with self build mortgages did I see a house built within the original budget and there was a project manager employed on that job as the owner was living in UK. More often than not top up mortgages were required.
Invariably costs go over for different reasons, one as stated above is that unexpected things crop up. Another major one is that 'nicer' fittings and fixtures are chosen than was originally budgeted for, for example the skirting boards are going in and the deeper fancier one is much nicer but more expensive, the 'sure we'll only do it once' attitude kicks in and before you know it that applies to the taps, bathroom fittings, doors, extra sockets (because there is never enough) etc etc etc. Not to mention when you start on the kitchen.
That I have found was the greatest cause of gradual overspend rather than one big building expense such as hitting rock or similar.
I would think 10% was cutting it fine unless you are both very disciplined with extras.