the documents a mortgage lender requires for a self build are :
confirmation site is owned by you or confirmation of site purchase price
copy of planning permission
Architects/engineers letter of supervision ( lender will supply this)
Quote for cost of construction. ( your own estimated cost will not suffice)
you best approach is to approach a mortgage lender for Approval in principle , you will then know how much you qualify to borrow, if this covers your budget well and good, if not , back to the drawing boad and reduce costs, ie landscaping, bathroom accessories.
mortgage lenders will typically, subject to income, approve you for 90% of the total cost to purchase site and construct property. .. if you own the site then you can borrow more for construction as the total mortgage amount relative to the value on completion will be lower than if you needed to borrow to
purchas site also.