”房客is permited by Landlord to replace front lawn at their own ex

A bit tricky.

"at their own expense". At whose expense? the landlord's, the tenant/s?

I understand that the subject of the sentence above is "房客“, but "房客“ appears not be the direct subject of the sentence in the original agreement.

Even if the lease agreement states "Tenant is permitted by Landlord to replace front lawn at their own expense". It is still in murky water.