A sublease is an agreement where someone takes over part or all of an existing lease. This type of lease involves at least three parties.A sublease agreement is a legal contract signed between a tenant and a third party allowing the latter to rent out the entire property or any part of it. How Much Does a Sublease Cost? The average cost (i.e. Subletting, also known as subleasing, involves renting out a property or a portion of it that you are currently leasing, to a new tenant. When a tenant asks its landlord to consent to a sublease the Landlord will rightly focus first on business concerns. Who is the subtenant? If you want a sublease agreement, you must do thorough homework. Much like apartment renters use Airbnb to rent out their extra rooms, businesses can sublease office space for extra money.