Just a word of caution about delaying the draw-down of an agreed mortgage.  To part-fund our build I needed to borrow around £100k.  Rather than get a self-build mortgage (because they are expensive and attract massive fees from the few brokers that specialise in them) I opted to mortgage our existing house (we had no mortgage on it, and intended to sell it after the build anyway).  I shopped around, and Santander, who happened to be our bank at the time, agreed to lend £100k without any problem
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