Tenant is allowing guests to stay with him

I have a studio flat attached to my main residence house.Electricity and water supply for the studio is through the main house.Hot water and heating is also through the main house . There are No gas appliances inside the studio.
All experts/landlords are kindly requested to answer these questions:
1)Do I need to provide the tenant with a Gas Safety Certificate?
2)What other certificates I should provide?

Most important question:

3)The TenancyAgreement is for one person.BUT the tenant keeps inviting his girlfriend to stay with him for 1 ,2, or recently for 5 nights.What should I should do especially that in my home insurance I have mentioned one tenant only.Also I have noticed there was more water and electricity use (almost 3 times our usage)when there are people staying with the tenant in the studio.

This is a more involved question than it might appear.

As the property is attached to your home, the occupant would probably not legally have an Assured or Assured Shorthold tenancy. If the tenant is sharing any space with you, (kitchen/bathroom/lounge etc), then they would have a licence to occupy as a lodger. If not, they would have a non-assured tenancy. In both those cases, there is no legal requirement for you to have/serve the tenant with a gas safety certificate. If you were unaware of this and have issued the tenant with an AST then you are contractually bound by any terms in that agreement. This may or may not include having a gas safety certificate. In this case if there are any gas pipes or meters that run through the studio, then you would certainly need to have a gas safety certificate, or possibly, depending on the degree of separation of the studio.

I say probably, because it can be a grey area as to whether an annexe counts as part of the landlords home for the purposes of the legislation and if you’re in any doubt, you should check with a solicitor.

Its perfectly reasonable for the tenant to have guests stay over and a guest is not a permanent resident and should not affect your insurance. You may want to negotiate a small increase in the rent to cover the increased use of utilities.

Thank you David that was interesting to someone like me with no previous experience/knowlege in letting a property.

The agreement will have a guest clause. I would advise that you read what the terms are and remind the tenant of such. As David suggests I would charge extra for the guest…
You may need to consider an EiCR EPC and how to rent guide if you have served an AST in place.

That’s a concern as there are legal requirements you may have missed. The penalties for mistakes in property letting are now very serious so I would suggest you undertake some training to help avoid these.