If not at the time in default under this Agreement, Lessee, at its sole expense, may make reasonable alterations of and additions to the Improvements or any part thereof, including the rebuilding and reconfiguring of the towers, provided that any alteration or addition # shall not change the general character of the Real Property, or reduce the fair market value thereof below their value immediately before such alteration or addition, or impair their usefulness, # is effected with due diligence, in a good and workmanlike manner and in compliance with all legal requirements and insurance requirements, # is promptly and fully paid for by Lessee, and # is made, in case the estimated cost of such alteration or addition exceeds Ten Thousand Dollars ($10,000), under the supervision of an architect or engineer satisfactory to Lessor and in accordance with plans, specifications and cost estimates approved by Lessor, which approval shall not be unreasonably withheld, conditioned or delayed.