Query on assessment

in one of the ammendment it is said that the AO cannot open cases which are more than 7 yrs old and hence if we have a building which was purchased 20 yrs back can b introduced in the books via capital a/c and u r not answerable regarding the source of income. what will v reply if the AO asks us for the reason of not showing the building in books of the presceding 6 yrs while it was still in r ownership. Y dint v diclose it till today?