This was our build finance story.
We owned a nice newish 4 bed and had paid down the mortgage substantially, also had some savings.
When we stumbled across our new house (tired 1950's detached on 1/2 acre garden) we re-mortgaged the original property to the hilt and used the equity to help buy the new (old) house. Then rented out the original house to cover that mortgage and started planning our build.
A few years later, we eventually sold the original house and had the capital (our build fund) in the bank. No need for a self build mortgage we thought.
However when we informed our lender that we were planning 'substantial' works on the house (i.e. demolishing and rebuilding) they got cold feet and after much faffing, released us from the mortgage penalty free - however that ate much of the build fund.
We then got a self build from Ecology, who were very easy to deal with but have strict criteria on lending to passive or SAP A projects.
They were happy to give us the whole chunk upfront or release it as we needed it, there were no inspections or surveys to release funds.
On our self build team, Mrs B is the visionary and I am the one who figures out how to do it. Works quite well and irritatingly she's almost always right with her decisions.... (even after I've said about six times - thats impossible, we cant afford that, that will look stupid etc...)