You propose whatever conditions you want, and formalise it. You could give approval subject to reviewing the plans and inspecting the workmanship, or approving the builder, or whatever.
You can also make it a condition that the patio has to stay when they leave, or - if you don't like it or it's poor quality - that they have to remove it.
I don't believe that if the tenant pays for it that it could be a tax deduction for you, but hey, I'm far from a tax expert and I've heard of stranger things, so I'll leave that one for one of them!