Hi,
The £2500 limit is for when a tax return is required. this is shown on the help tool at
check if you need a tax return.
The £7500 limit is for rent a room relief. if you are looking to claim this then you cannot claim any other expenses in respect of the rental income received.
If claiming rent a room relief, you also need to show this in a tax return and applies to a room on your own house. you cannot use it if you let out a whole property.
Thank you.