“(11) Not to alter Not at any time ....to make structural alterations in or additions to the plan elevation or appearance of Demised Premises or in any of the party walls or the principle [sic] or bearing walls or timbers or iron steel or other supports thereof nor to alter connect to extend or otherwise interfere with any heating or other plumbing installation nor shall the Lessee do or fail to do any act deed or thing which would adversely affect the support, repair, maintenance, cleanliness or enjoyment of the flats in the building.”