I would imagine 80% of purchase price of site is the max you can get and even that is not done by all lenders. However, if you are buying it without outline planning then it is not a site, its a field and would be hard to get mortgage on. Other alternative is to pay deposit and make balance subject to planning then once the planning is issued you can apply for the loan, if vendor is agreeable. Usually possible then to get 100% of building cost assuming total cost is less than 90% of proposed value of completed property.