Most lenders will walk away from an agreement in principle if you now have a new job and start a new probationary period. The agreement in principle is an indication that a lender will offer you money based on the information you have told them - this information has now changed. There are examples such as Cheeky's where a lender will use their discretion and allow the application to proceed. The Nationwide will ignore a probation period and will lend to people who have just started a job. Speak to amortgage adviser to get the best advice.