(1) Definitions:

(a) "Airspace" is that space located above, at or below the highway's established gradeline lying within the approved right of way limits.

(b) "Department" is the Washington state department of transportation.

(2) Any use of such space shall be subject to approval of the Federal Highway Administration.

(3) Any use of such space shall be subject to compliance with all applicable city, town or county zoning requirements.

(4) Any application to the department for the lease of such space shall describe in detail the use to be made of such space and the physical facilities to be installed and maintained on state right of way.

(5) The lessee shall be solely responsible and shall hold the state harmless for liability for any and all damage to persons or to public or private property that may result from or be caused by the use of such space or from the erection or maintenance of any structure or facility upon the highway right of way. The lessee shall be liable to the department for any moneys expended by it for the protection or repair of any state facility required as a result of any such use.

(6) The lessee shall be required to carry liability and property damage insurance in amounts required by the department.

(7) No use of such space shall be allowed which subjects the highway facility or the public to undue risk or impairs the use of the facility for highway purposes.

(8) Use of such space shall be covered by a properly executed airspace lease.

(9) Consideration for occupancy:

(a) Where the airspace can be developed and used as an entity the consideration shall be economic rent.

(b) Where the proposed use of the airspace is in conjunction with an abutting tract, rent shall be based on its contribution value to the abutting property but not less than economic rent.

(c) When the use of the property constitutes a highway purpose the rent may be offset in part or in whole with other valuable considerations as determined by the department.

(10) The granting of any use of such space shall be subject to the discretion of the department and upon such terms and conditions in addition to those stated herein as it shall deem proper.

(11) No assignment of any lease by the lessee shall be of any force and effect unless prior written approval of such assignment has been given by the department.