Fluffymub,
There are too many unknown variables.
Age, kids, other loans, rental income?
If you could do a 30 year term, have no loans, secure jobs, well managed current accounts, no children and if the rental income would cover 1.5 times your current mortgage repayments stress tested at capital and interest payments of 5% then this would probably be a runner.
Bear in mind, it is still tough to get investment mortgages approved unless they are rock solid.
You would also need savings to cover overruns on the build of roughly 15% of the build cost.
The loan to value on your current house will need to be < 70%.
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