Pension credit and renting my home?

Pension credit and renting my home?
Added (1). My parents live in there home and are thinking of moving into my house as its a large house. They own there home outright, currently they are claiming pension credit and normal pension. My question is if they rent out there own home and come to live with me would they need to declare the rental income to hmrc as they would be contributing some of the rent towards my mortgage. They only own 1 property which they live in.

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