One thing to consider with sqft is that the MLS may have different rules for submission of above ground sqft than appraisers. There is also usually a disclaimer at the bottom of each page indicating all information may not be correct. Speak to your agent if you are concerned.
This website may provide more information: http://www.michigan.gov/documents/Vol1-02GeneralInstructions I believe you will notice, little measuring of interior walls is involved. (source State of Michigan) You could follow up on some appraiser websites and ask them their opinion.
As to - "I guess I have to hire someone to do a measurement." I would agree. Whether you are getting a mortgage or paying cash an appraisal may be well worth the money.
I've recently found that what's listed on the Public Record Data, the square footage noted by two different appraisers on the same home were all different (on a closing last week).
Simply put, there's no "definite" and concrete standard for the footage calculation apparently.
Is there a specific reason you feel you need this?
It's a simple matter to obtain copies of your home's original floor plans from the City's Planning or Building Departments. However, if you wish to get detailed information (blueprints, for example), you will have to OWN the building, and will normally need to obtain the permission of the original architect to release the plans.
Unfortunately, however, even having the copies of the floor plans for the home will not necessarily provide you with the "exact" square footage of the property. In fact, building plans on file with your City's Planning Commission or Building Department are seldom, what we call in the industry, "as builts." More likely, the plans will be close, but I've found that they can be off by a couple hundred square feet.
The best means to determine the exact square footage of your home is to contact a professional surveyor to assist you. Regardless of the housing floor plan, only a professional can determine the home's exact size and can provide you with the report that you'll need to have those records corrected at the City.
Grace Morioka, SRES, e-Pro
Area Pro Realty
San Jose, CA
For your reference, most appraisers don't use price per sq. ft much anymore when doing comps.
Even so, they'll use a real low % value for the difference in price justification for that said difference.
Times are a changin' and appraisers are quite scared to give adequate value, in my opinion.