heizitu ni site ha, siyakusyo ga konde iru.

にしては means something like for, considering, or given that, and it often carries a nuance of unexpectedness.

So:

平日にしては、市役所が混んでいる。
= For a weekday, city hall is crowded.
= Considering it’s a weekday, city hall is surprisingly crowded.

The speaker is saying that the situation is a little unusual compared with what they would normally expect on a weekday.

Because にしては is a set grammar pattern.

• 平日は、市役所が混んでいる。
  = As for weekdays, city hall is crowded.
  This sounds like a general statement about weekdays.

• 平日にしては、市役所が混んでいる。
  = For a weekday, city hall is crowded.
  This means it is more crowded than you would expect for a weekday.

So にしては adds the idea of comparison with a normal expectation.

In this pattern, にしては works together as one grammar expression, so it is best to learn it as a chunk.

You do not usually need to analyze the に by itself when learning this structure. The important point is:

• Noun + にしては
• meaning: for, considering, given

Here, 平日 is a noun, so it becomes:

• 平日 + にしては

This is very common with nouns:

• 子どもにしては大人っぽい。
  = For a child, they seem mature.
• 初心者にしては上手だ。
  = For a beginner, they’re good.

Usually, yes—or at least mild unexpectedness.

It often means:

• given X, I would expect Y
• but the actual situation is different from that expectation

So in this sentence, the speaker probably expects city hall to be less crowded on a weekday, but in reality it is crowded.

It does not have to mean strong shock. It can just mean:

• more than expected
• less than expected
• different from what is normal

Because 市役所 is the thing being described as crowded.

• 市役所が混んでいる
  = City hall is crowded

Here, 混んでいる is describing the condition of 市役所, so が marks it as the grammatical subject.

If you changed it to は, it would sound more like a topic:

• 市役所は混んでいる。
  = As for city hall, it’s crowded.

That is possible in some contexts, but が sounds natural here because the sentence is mainly presenting the current situation.

混む means to be crowded.

Its -ている form, 混んでいる, usually describes a current state, not an action in progress in the English sense.

So:

• 市役所が混んでいる
  means City hall is crowded

It does not usually mean something like City hall is becoming crowded right now.

This is a common pattern in Japanese. Some -ている forms describe a resulting state:

• 結婚している = is married
• 知っている = knows
• 混んでいる = is crowded

The comma shows a pause after the opening phrase:

• 平日にしては、…

This is very common when a sentence begins with a phrase that sets the context. It helps separate:

• the standard of comparison: 平日にしては
• the main statement: 市役所が混んでいる

The comma is helpful, but not absolutely required every time:

• 平日にしては市役所が混んでいる。

That would still be understandable.

They are similar, but the nuance is a little different.

• 平日にしては、市役所が混んでいる。
  = For a weekday, city hall is crowded.
  This compares the situation to the normal standard for weekdays.

• 平日なのに、市役所が混んでいる。
  = Even though it’s a weekday, city hall is crowded.
  This sounds more directly like that’s contrary to expectation.

So:

• にしては = comparison with a standard
• なのに = stronger feeling of even though / despite that

In many contexts, both can work, but にしては feels a bit more like judging by the category, while なのに feels more like this fact should have led to a different result.

No. It is very common with nouns, but it can also follow other forms.

Some common patterns are:

• Noun + にしては
• Verb casual form + にしては
• い-adjective + にしては
• な-adjective + な + にしては

Examples:

• 日本語を習ってまだ三か月にしては、上手です。
  = For only having studied Japanese for three months, you’re good.

• 高いにしては、このパソコンは便利だ。
  = For being expensive, this computer is useful.

• 静かだにしては is not right
  but
  静かなわりに is more common there

In practice, learners most often first see にしては after nouns and clauses.

The standard readings are:

• 平日 = へいじつ
• にしては = にしては
• 市役所 = しやくしょ
• 混んでいる = こんでいる

So the full sentence is read:

へいじつ に しては、しやくしょ が こんで いる。

One pronunciation note:

• 市役所 is しやくしょ, not いちやくしょ
• 混んで comes from 混む（こむ）, and the む changes to ん in the -て form

Usually it is translated as city hall, but the nuance is closer to the city government office / municipal office.

So in English:

• city hall is the normal translation
• but depending on context, municipal office may be more precise

In this sentence, the important point is just that it is a public office building that people go to for administrative matters.

Yes, as often happens in Japanese, the broader context is omitted.

The sentence does not explicitly say:

• today is a weekday
• I’m looking at it now
• I expected it to be less crowded

But all of that is understood from context.

A fuller English-style interpretation might be:

• Considering that today is a weekday, city hall is crowded.
• It’s pretty crowded at city hall for a weekday.

Japanese often leaves those background ideas unstated when they are obvious from the situation.