Why though? I could understand somewhat if you got a mortgage based on having a lodger but it doesn't make the slightest bit of difference to them what you do with your house once the loan & repayments are sorted.
Just another way for them to get more money out of you for nothing.
No way of them finding out, so don't tell them.
But thats my point, if you need to claim on housing insurance then there is a likelihood that they will find out.
I don't agree with them getting extra money for nothing, it's just how things work!