Learn Hardcore Finnish: Kotini on ylin kerroksessa.

Kotini means “my home.” The suffix -ni is the first-person singular possessive suffix, equivalent to my in English. You attach it directly to the noun koti:
koti + -ni → kotini
Without -ni, koti would simply be “home” with no owner indicated. You can also say minun kotini, but the suffix -ni is still required in standard Finnish.

Why is the verb on used here instead of a preposition like “in” or “at”?

In Finnish, location and existence are expressed by the verb olla (“to be”) plus case endings on the noun, rather than separate prepositions. The word on is the third-person singular present of olla and covers both “is” and the notion of being in/at/on something. The specific sense (in vs. on) comes from the case ending on the noun (see kerroksessa below).

Why is kerroksessa in the -ssa/-ssä case instead of kerroksella?

The ending -ssa is the inessive case, used for static location meaning “in”. Although English says “on the floor,” Finnish treats a floor level as an interior zone, so you say kerroksessa (“in the floor”). The adessive ending -lla/-llä (as in pöydällä, “on the table”) expresses being on top of a surface. With floors and rooms, Finns normally use the inessive (-ssa/-ssä) to indicate “in/at that floor.”

What is ylin, and how is it formed from ylä?

Ylin is the bare superlative form of ylä (“upper/above”), meaning “topmost” or “highest.” The degrees go:
positive: ylä (“upper”)
comparative: ylempi (“more upper/higher”)
superlative: ylin (“uppermost/highest”)
The bare superlative ylin can stand in front of a noun in its basic (nominative) form, e.g. ylin kerros = “top floor.”

Why isn’t it ylimmässä kerroksessa, and what’s the difference between ylin and ylimmässä?

When you fully inflect the superlative adjective to match the noun’s case, you take the superlative stem ylim- and add the inessive ending -mässä, giving ylimmässä kerroksessa. That is the more formal, fully inflected version. In everyday speech and informal writing, however, Finns frequently use the bare superlative ylin together with an inflected noun (ylin kerroksessa). Both Kotini on ylimmässä kerroksessa and Kotini on ylin kerroksessa are understood as “My home is on the top floor,” though the former sounds a bit more bookish.

Can the word order be changed, for example Ylin kerroksessa on kotini?

Yes. Finnish has a fairly free word order. Placing ylin kerroksessa at the front emphasizes the location:
Ylin kerroksessa on kotini.
= “On the top floor, my home is.”
The core meaning stays the same.

Could I say talossani or asuntossani instead of kotini?

Yes. You can swap koti for other nouns with the first-person suffix to change the nuance:
talossani (“in my house/building”)
asuntossani (“in my apartment”)
Koti stresses the idea of home as a personal place, while talo is simply the physical house and asunto the apartment unit.