Found an answer here HTML: http://www.runuo.com/community/threads/houses-more-then-one-for-1-0-please.84398/#post-721532 ArteGordon knows his stuff! Works perfectly! Easy to understand and modify!
The overall advice is sound. You loop through each character on the account so you determine how many houses the account has in total.
Line 3076 in BaseHouse.cs Code (text): for ( int i = 0; i < list.Count; ++i ) change the 0 to a 1 for 2 houses, 2 for 3 houses and so on. This is from a 2.1 tester.