Adding a second pet on request from tenant

Other

I've got a single tenant with 2 young children and a small dog, who have been in the property for 7 months.. last month she asked my permission to move her boyfriend in, which I've consented to and she's signed the addendum for an additional 'permitted occupier' and Right to Rent statement etc.. and now asking if they can bring a second dog in? A staffy which will be registered as an anxiety dog.. 
My concern is when I did the 6 month inspection the house had a lot of 'stuff' in it and so not sure how suitable it would be for another dog.. has anyone had this type of issue before? Am I overthinking it?! She's always paid the rent on time, and is asking permission first which is a good sign, so I don't want to rock the boat as such! 
Any advice appreciated!