Would you include the GLA of an indoor heated pool within the GLA area if the pool is located on the lower walkout basement area?
The Fannie Mae GLA specifications are only guidelines and can be adjusted to reflect what is most common in your market area.
One GLA guideline is that only areas of the dwelling that are 100% above ground and heated and finished like the main part of the house should be included. I doubt the drafters of the original guidelines were thinking about indoor heated pools.
The important thing to remember is that whatever you do, you need to explain in detail in the appraisal so that the users of the appraisal and reviewers who has access to some other source of the property GLA measurement will not confused by the difference. This will also help explain the basis of any adjustments to the comparable sales based on the difference in GLA.