I am in a similar situation to you. I have found a site, hoping to submit planning permission application in the next month which will mean we will have to pay €200K for site when pp comes through.
I have a house worth about €240K with mortgage remaining on this house of €130K. The bank have offered an equity release of €167 on my house.
Have you spoke to the bank about another mortgage for the site?
We have to come up with 10% ourselves as banks / BS won't give 100% mortgages for sites - we are lucky to get 90% with EBS.
Also I am taking out a second mortgage with a partner and for some reason they are not too concerned with my existing mortgage. I will be keeping this house and renting it when the new house is built.
You may have an option of paying interest only on the site - we have 6 months interest option.
We will be under a lot of pressure to keep both going but hopefully it will be well worth it.
Talk to a mortgage adviser - they are used to this situation. I found Mortgages Direct very good.