Renovation mortgage

Does anyone have any insight on how easy or difficult it is to get a renovation mortgage, ie a mortgage that covers the purchase and the renovation costs? And how does one get such a mortgage? You identify a ‘wreck’, have a quote for costs of work to complete a renovation, get approval from the lender? And if the renovation costs rise unexpectedly do you need to have agreed some kind of top up loan at the time you take out the original mortgage? I thought this would be quite difficult to arrange but I looked up one wreck I’d passed on some months ago due to it seeming to be cash only and found that the buyer had been able to arrange such a mortgage fairly rapidly.