Little bit of difficulty understanding this...
You're building a house on a site you don't have title to. Where exactly would the security be for the lender? They could end up with a house (or part of one) on a plot of land owned by somebody else.
The owner of the land is prepared to sign a declaration that the plot is yours.
That's all very well but by the sound of of it half of it is actually his.
Mortgage providers are a bit old fashioned - they tend to insist on things like clear title.
I daren't even ask about planning permission.