I would guess that a top up depends entirely on the valuations on the current house and the land you would like to buy, your risk profile and repayment ablilities etc etc etc.
These days the banks seem to be reluctant on any form of mortgage lending (despite what they say). Last week I tried to put €10k onto my mortgage and was infuriated to be informed that I would have to go through the whole mortgage application process again and in any event - they would only do it for €20k or more.
However, they would have no problem giving me a personal loan with higher rates and shorter terms. This also helps them hit their 'new loan' targets. Drove me nuts!