If I'm reading your query correctly, you don't need to use the current house as collateral. For a self-build project, banks will lend a percentage of the site cost (75%+) and a percentage of the build cost.
You will need savings and to be able to prove affordability. Stress-tested repayment on, say, €280,000 borrowed over 25 years would be €1,800 per month.
I'm assuming you're paying no rent at present. I'd suggest that you start saving €2,000 per month for the next year and then you should be ready to go. I'm assuming you have no other loans. If you have, clear them from savings and then get saving.