No, refinancing or your mortgage has nothing to do with it.
You would start with $27,000, and add the cost of any improvements that were NOT replaced. For example, if you paid $1000 for a new roof in the 1970's, and then you replaced the roof again in the 1990's for $5,000, you would only add the 'newest' improvement (the roof for $5000).
The most common improvements include roof, furnace, central AC, remodeling, additions, etc..
