Office landlords charge rent and state the square footage of office buildings based upon the "gross leasable area" of the office building and of the tenant's premises. Sometimes, that causes tenants difficulty, as they mistakenly think that their space actually contains those measurements. In reality, the gross leasable area of a building includes common areas, elevators, common bathrooms, stairwells, and other portions of the building that the tenant doesn't occupy. The actual square footage of the tenant's space is called the net rentable area of the space. Almost without exception, the gross leasable area of a tenant's space is larger than the actual physical measurements of the space. Thus, that 5,000 sq. ft. of office space that you are looking to lease isn't really 5,000 sq. ft. It's more like 4,300, or 4,500, or 4,700 (it depends on the particular building and the amount of non-rentable space that is included in the figures).
I've seen tenants do drawings of how their company is going to fit into a space. They are often surprised, and say "I could have sworn that 5,000 sq. ft. would have done the trick. In many cases, that's because your 5,000 sq. ft. space isn't really 5,000 sq. ft.. When you are budgeting for your new office space, be sure to take into account the difference between the gross leasable area of your space and the net rentable area.