It is rather unfortunate to disregard value-enhancing modernizations and renovations, as these increase the value of the property and thus, by using equity capital, reduce the loan-to-value ratio. I would always recommend making the bank aware of the measures when purchasing the property and applying for the financing holistically.
One option is to estimate the costs roughly / have them estimated and to borrow a little more, build in a buffer, and choose a bank that accepts a non-utilization amount, meaning that parts of the loan do not have to be drawn without incurring a non-utilization compensation.
It is also possible to consider less equity capital in order to be able to react flexibly to additional costs and to reduce the loan, the principal, at the end of the construction period by using any surplus as a special repayment.
In principle, the following questions arise:
1) How accurate is your cost estimate?
2) Are the services to be performed by tradesmen sufficiently clear?
3) Is there no construction expert or architect available in the vicinity?
If a builder is clear about the costs, or has certainty, or includes this in the loan, then our in-house architect will prepare a cost breakdown free of charge that banks will accept. But just by the way.