The erection of mansion houses and such other usual and necessary buildings, outhouses, and offices as are commonly appurtenant thereto and held and enjoyed therewith, and completion of mansion houses and such appurtenances as aforesaid, and improvement of and addition to mansion houses and such appurtenances as aforesaid already erected, or the improvement of and addition to houses which are. capable of being" converted into mansion houses suitable to the estate on which they stand, so as such improvement and addition be of a permanent nature, provided the mansion houses so erected or enlarged or converted are suitable to the estate on which they stand as residences for the owners of such estate, shall be improvements within the meaning of the " Improvement of Land Act, 1864."