There is no simple catch all on what you should do. It would depend more on the home. Can you give us more details?
It's tricky...but sometimes that's the best you can do!
My point is this - focus on the value of the neighborhood, not the structure. It's easy to calculate the delta of value between properties with "X" versus "Y" bedrooms and baths. If you can establish the value of the land, it's a downhill ride to calculate the replacement cost of the structure and arrive at a total.
This may not not work in your area, and you may want to use different methods depending on type of property. For example if you have a 5 or 6 bedroom property you probably aren't going to find a lot of comps so you might look at 4BR+ and extrapolate. Your broker should be able to help you with this.
Lance King/Owner-Managing Broker
Bernard Gibbons, J. Rockcliff Realtors
DRE License # 01331583
Phone (925) 997-1585 - email@example.com
It's hard with values changing for the better. I also prepare my sellers that the comps may be 3 months behind on seeing a boost in values.