Another reason is that during the time of the works the value of the property could be significantly reduced (say, in for example, a rear extension being demolished then rebuilt), therefore risk exposure for the lender increases regardless of the outstanding loan amount. Remember if you have a mortgage, the property belongs to the bank in reality, not you, so you're altering their property. If YOU'd lent money against a property and for several weeks that property would have no roof so you'd potentially be under-secured against that loan, you'd probably be interested and most likely would ensure there's a clause relating to that in your lending terms. Given that you'd not have the property without a lender's financing, it's surely reasonable to oblige and let them know your plans. To not do so is unreasonable. As mentioned already it's usually a formality - what's best is when you've set the timeline, costs and plans, all PP received and you're ready to go. That way you can answer questions from the lender with confidence and minimum disruption to your proposed works. With a solid plan any changes/negotiations with the lender are easier to settle. People frequently criticise banks for under-capitalising against debt they issue, but will intentionally mislead a bank in this way, helping to create a situation (albeit for only a few weeks) where the banks may be exposed over their regulatory entitlement.