You should have a formal mortgage offer from a lender before you sign contracts - your solicitor will require this as it is a written guarantee that the funds will be in place and that you are capable of completing the contract. Should you for any reason fail to secure a mortgage after contract signing, you will forfeit your deposit and you are also liable for your solicitors fees, the builders solicitors fees and all other costs borne by the builder due to you reneging on the contract. In summary, it's not mandatory but you should get it sorted now, by the time you get your documentation together and submit your application it should take about two weeks to turn it around. I wouldn't panic at all, just get your skates on... also if you need a little extra time, your solicitor should be able to negotiate with the builders solicitor for an extra few days.