Of course it adds value to the home - a home the OP owns. Home costs in many areas have risen quite a bit in the past 20 yrs. (Let's assume it took at least a year to plan and build it.)
Appleplie, how many square feet is her and your space that was added? I assume she has a full bath? Kitchen? Bedroom? Living room? What else? Any outdoor space like a deck or porch? Attic? Cellar?
Is it possible to still make it a legal apartment, even if it isn't now? (Often you can legally convert an interior space for a legal apartment, but you cannot add on to do it for a certain number of years.)
People rent out both legal and illegal apartments all the time, and many people have extended family living and au-pair/nanny situations, so I don't think we should assume that that space would not be wanted by anyone.
OP, as I understand it, paid taxes on the home, which presumably increased once the addition was built. Those could've been significant, times 19 years, so the OP is saying that those could've negated the money that DM initially invested in the home. In all likelihood it could've well evened out.
For many, the issue is less a financial dilemma at this point than it is an ethical one. (ETA and clearly there are two schools of thought on that!) Did DM rely on the belief that she would have a home with her daughter for a lifetime? Had she known she was going to be asked to part ways, would she a) have invested her money in this home in the first place or b) lived a lifestyle that did not require saving money in order to pay for a future of "independent living"?