When applying for a mortgage, it is advisable where possible to ensure that your overall circumstanced are stable. Lenders like stability because it represents low risk. If you have been in stable employment with a steadily rising income for a long time then lenders can be reasonably confident that you will be able to meet all of your commitments so long as no unforeseen disaster strikes.
Changing jobs within the same company is unlikely to be a problem as you have a history with that company and your employer will usually be willing to confirm any salary changes to your lender. Similarly, moving jobs to a new employer may be acceptable but many lenders will require you to have worked in the new job for a period of time while some may accept a letter from your employer confirming the appointment. Much will depend on the lenders view of your employers standing.
If you are being forced to change jobs through redundancy or for health reasons and have not yet secured new employment, lenders will be unlikely to risk giving you a loan. Under these circumstances it would generally be deemed to be unwise for someone to take on a credit commitment when their income is known to be uncertain going forward.
Taking Out an expat mortgage with a new job
There is no doubt that lenders prefer to accept new borrowers who have been in a steady job for a reasonable period of time as this allows them to be more accurate with their assessments on the borrowers ability to repay the loan. They will also look at the borrowers employment history to see if changing jobs is commonplace with the borrower, making them a riskier client. Provided that there are good reasons for the job change and the income from the new job meets the lenders requirements any problems can usually be overcome.
The salary income from any new job is what will matter to the lender but if that income is made up from a base salary plus commission payments, bonuses or overtime working, you may find that the lender may not accept these additions as part of your salary and may reduce them or delete them from your total income which will in turn reduce the amount you will be able to borrow.
If you have recently moved from a salaried job to self-employment, this could be a problem as most lenders will require you to be able to show at least two years worth of tax returns to prove that your income is stable. Unless you can show proof of your earnings over the required time period, your application for a mortgage will most probably be rejected.