there is covered area can it be calculated in usable area

* Usable area refers to the area within the suite that the tenant has exclusive use of. * The rentable area is greater than the usable. It takes into account a share of the common areas of the building that the tenant shares with other tenants. These areas include a share of corridors, lobby and washrooms that other tenants use. * In simple terms, the usable area is the size of the tenant's private space while the rentable area is the area on which rent is charged. The landlord has the right to recover rent on all the area in the building including shared spaces- corridors and washrooms are not free!

Of course, if you can use that space.

usable water

Yes, usable is an adjective to describe a noun as able to be used; for example usable information.