The dead 20 years later

Mostly skeleton is my guess, but the oldest bodies I've had to look at were not 20 years undisturbed in a bldg. Shallow grave, etc.

For your scenario, humidity plays an important role, esp in the first few years, on whether body will putrefy (need a sponge) or mummify (nice and dried out). However, it is hard to imagine an abandoned building being rat/mouse-proof. And over 20 years, it's quite a banquet. There are many small bones for the little critters to haul off. Even if the body was fully clothed, the mice should be able to deal with the boots in the course of 20 years.

For trace matter, decide now if the surface the body is on is or is not permeable.

And even if it was somehow rat/mouse-proof, it probably wouldn't be insect-proof. After 20 years in a location where bugs could get to it, you'd have skeletonized remains with perhaps small amounts of dessicated tissue remaining.

What do you mean by "protected from scavengers"? Even if the body was sealed hermetically from insects, the warm temperature periods would still allow the gut microbes to consume the flesh.

You also have to consider the human factor. Nowdays any abandoned building isn't likely to remain abandoned for long. It would shortly be occupied by homeless people or drug dealers or both. 20 years is an awfully long time.

If the body's between the walls after 20 years you may have imprints of decomposition around the bones. Clothing and jewelry would be left too unless the body was dumped nude.

I'd like to recommend this book, which I found in the library this week. It's written by an American for American writers which presents a problem when it comes to translating the legal shenanigans into their Scottish or Canadian equivalents - but it might be a help to you. (Link hidden behind the cover photo).