Jeff,
I would suggest that if your are forced to do a cost approach on this home that you plan on disclosing in your reconciliation that you were requested to do so and that you give it no weight, just like Pamela said.
Next, I would recommend you review Marshall and Swift's Depreciation Section. If you are going to use M&S as your primary source of cost data and depreciation analysis, you should be comfortable with their current methodolgy. If worse comes to worse and you are facing off with some attorney in the future, it would be a good thing to have the current methodolgy on your side rather than searching for ways to get around it during examination.
In M&S, you will find that the use of their depreciation schedules is based on a slightly different theory of loss than the one that you were probably trained in. They refer to is as an "extended-life theory of loss, based on the concept that a proven ability to exist leads to continued existence". And yes, that was a quote from the instructor's manual published by M&S for the use of their residential cost guide.
Anyhow, the current method for determining depreciation is based on the following equation:
Economic Life - Remaining Econimic Life = Effective Age.
This is different than the way most of us were trained:
Economic Life - Effective Age = Remaining Economic Life
The newer methodology works slightly different and takes into consideration not just the physical depreciation factors but also some external factors (what is happending in the subject neighborhood, for example) as well.
In your case, the book will probably say your home has an economic life of 60 years. That's a pretty arbritrary number, but it's the one they use. In your market segment, you may find reason to estimate that this structure, if maintained under an average maintenance scheme, will probably continue to have contributory value for another 40 or more years. This would result in an effective age of 20 years using the above methodolgy (60 - 40 = 20). Then you use the table to find out that the depreciation amounts to 18%. If you decide the remainder is indeed 50 years, then you will come out with a 10-year effective age and a lower depreciation rate. The table does not count any undue functional aspects that you would find in a 112-year old home, like floorplan, inefficient construction techniques, dated appliances and fixtures, etc. You would have to add in more loss for these items, if present.
Since you didn't think the cost approach woule be reliable for this home in the first place, there would be no reason to get excited if the results of this approach are completely different from the other approaches to value. The different result merely demonstrates your point. Personally, I would consider not doing the cost approach within the main body of the appraisal and offering it as a supplemental. I detest clients who want to dictate appraisal theory and technique, especially when they are wrong.
George Hatch