The particle に marks the destination or point of arrival with verbs like 着く.
So:
This is similar to how に is used for a target or endpoint.
Compare:
With 着く, に is the normal particle for the place where someone arrives.
着いた is the plain past form of 着く.
Here, it appears before ところです, and that pattern requires the plain form:
So:
Even though the whole sentence ends politely with です, the verb before ところ stays in the plain form.
That is why 着きましたところです is not correct here.
In this sentence, ところです means something like at the point/stage of.
The pattern ～たところです means:
So:
This is a very common grammar pattern for talking about timing.
A useful comparison:
So ところ here does not mean a physical place.
Not literally.
By itself, ところ can mean place, but in grammar patterns like ～ところです, it means more like:
So in this sentence, 着いたところです is not the place where I arrived. It means:
This is a very important difference, because learners often first memorize ところ = place, but it has a wider use in Japanese.
Japanese and English express this idea differently.
Japanese uses the past form 着いた because the arrival has already happened. Then ところです adds the idea that it happened just now.
So the literal logic is something like:
Natural English often uses the present perfect:
But Japanese does not need a special present perfect form here. The plain past form plus ところです does the job.
Both can mean that you arrived at the library now, but the nuance is a little different.
今図書館に着きました。
= I arrived at the library just now / I’m at the library now.
今図書館に着いたところです。
= I’ve just arrived at the library.
The second one emphasizes that the action happened very recently, almost at this exact moment. It focuses more on the timing.
So ～たところです often sounds a bit more specific and vivid when you want to stress just now.
This is because です is attached to the whole expression 着いたところ, not directly changing 着いた into polite form.
Structure:
So the sentence is polite overall, even though the verb before ところ is plain.
This is very common in Japanese grammar patterns.
For example:
All of these are polite because of the final です.
Yes, very naturally.
For example, if someone asks where you are, you could reply:
This sounds like:
It is especially natural when you want to report your status right after arriving somewhere.
In casual speech, you might also say:
That drops です and sounds more informal.