I think one thing that is looked at when assessing a credit application are the number of other credit applications in the last 3 months and last 12 months. If you are planning to apply for a mortgage in 12 months I would suggest making any applications for extra credit cards promptly and then waiting a full 12 months before applying for the mortgage. Mortgage lenders presumably do not want to see boat loads of applications just before you've asked them for a large chunk of cash