Klagefristen er tre uker i henhold til underretningen.

Because Norwegian very often makes compound nouns by joining words together.

• klage = complaint / appeal
• frist = deadline / time limit

So:

• klagefrist = complaint deadline / appeal deadline
• klagefristen = the complaint deadline / the appeal deadline

This is very normal in Norwegian. Where English often uses two words, Norwegian often prefers one compound word.

The ending -en is the definite article attached to the noun.

So:

• en klagefrist = an appeal deadline
• klagefristen = the appeal deadline

Unlike English, Norwegian usually does not put a separate word like the in front of the noun. Instead, it often adds the definiteness to the end of the noun.

Here, frist is a common gender noun, so the definite singular form is fristen.

Because klagefrist is a compound noun, the ending goes on the whole compound:

• klagefrist → klagefristen

In Norwegian, when you say how long a deadline or period is, you often simply use:

• X er tre uker
• literally: X is three weeks

So Klagefristen er tre uker means the deadline lasts for three weeks.

This is natural Norwegian. English often prefers something like:

• The deadline is three weeks
• The deadline is for three weeks
• The appeal period is three weeks

Norwegian does not need an extra preposition here.

uker is the plural of uke:

• en uke = a week
• to uker = two weeks
• tre uker = three weeks

There is no article because numbers usually go directly before the noun:

• en uke
• to uker
• tre uker

So tre uker simply means three weeks.

i henhold til is a formal expression meaning:

• according to
• in accordance with
• pursuant to (very formal/legal English)

So:

• i henhold til underretningen = according to the notification
• or more formally, in accordance with the notification

This phrase is very common in legal, administrative, and official Norwegian.

Yes, very formal.

You are much more likely to see i henhold til in:

• official letters
• legal texts
• government documents
• administrative decisions

In everyday speech, people would often choose something simpler, such as:

• ifølge = according to
• etter = according to / under / following

But in official language, i henhold til sounds normal and standard.

underretningen is the definite form of underretning.

• underretning = notification / notice / formal information
• underretningen = the notification / the notice

This is a fairly formal word. In ordinary modern Norwegian, people often use words like:

• melding = message
• beskjed = notice/message
• varsel = notice/warning
• brev = letter

But in official or legal contexts, underretning is common.

Because it refers to a specific notification that both the writer and reader are expected to know about.

So:

• underretning = a notification
• underretningen = the notification

In this sentence, it means something like:

• the notification already sent
• the notice mentioned in the case
• the official communication in question

That is why the definite form is used.

Yes. A very natural alternative is:

• I henhold til underretningen er klagefristen tre uker.

This puts the formal reference first: According to the notification, the appeal deadline is three weeks.

Notice something important: when a phrase like I henhold til underretningen comes first, Norwegian still keeps the verb in second position:

• I henhold til underretningen
  • er
    • klagefristen
      • tre uker

This is the usual V2 word order rule in Norwegian main clauses.

It can often mean an appeal deadline in official or legal contexts.

Even though klage often means complaint, in administrative and legal Norwegian it can also refer to a formal complaint or appeal against a decision.

So klagefrist is often best understood as:

• deadline for filing a complaint
• appeal deadline
• time limit for complaining/appealing

The best English translation depends on the context.

Not really. It is clearly official, administrative Norwegian.

Signs of that include:

• klagefristen
• i henhold til
• underretningen

A more everyday-style sentence might be simpler, for example:

• Fristen for å klage er tre uker, ifølge brevet.

That means essentially the same thing, but sounds less bureaucratic.

So this sentence is perfectly normal Norwegian, just in a formal written register.