There are three sources we normally will use.... tax records, builder plans, or an appraisal measurement. All three can differ. There's all kinds of reasons why.
What I see often is that the builder files plans with the city, that's what goes on record and the buyer during the building process opts for a bonus room. Bingo...400sqft of actual space more than the county records.
Smaller nuances might be plan vs actual, maybe the slab got poured a little bigger or smaller than planned. Maybe the appraiser miscalculated by a foot or two or rounded up or down.
If you have a realtor already, ask them. If you don't call me and maybe I can help you determine it, once we know the specific address. - Wed May 23, 2012