How to build a new home but already have an existing mortgage?

Help! I'm am looking some Mortgage advice. My home is worth £130000 I owe £53000 on the mortgage.
I have the opportunity to buy a building plot and build my dream home (well a budget form of it anyway) the land costs £35000 and it should cost roughly another £110000 to build it. I have £50000 savings.
My existing mortgage company do not provide mortgages for new builds. Anyone know how i can go about getting it sorted?

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