Naručitelj radova plaća u mjeri u kojoj su radovi završeni.

Radova is the genitive plural of radovi. Here it works like English of the works.

So:

• naručitelj = commissioner / ordering party
• radova = of the works

Together, naručitelj radova means the commissioner/client of the works.

This is a very common Croatian pattern: one noun followed by another noun in the genitive to show a relationship.

In Croatian, the present tense is often used to state a general rule, especially in contracts, laws, instructions, and formal statements.

So plaća here does not necessarily mean is paying right now. It means something more like:

• pays
• is to pay
• shall pay

In legal or contractual language, this is very normal.

This is a formal expression meaning to the extent that, in the measure in which, or in proportion to how much.

Literally, it is something like:

• u mjeri = in the measure
• u kojoj = in which

So the whole phrase introduces a clause that explains the degree or extent of something. It is more formal than everyday alternatives.

Kojoj refers back to mjeri.

Here is the grammar:

• mjera = measure
• u mjeri = in the measure
• koja = which
• u kojoj = in which

Because mjera is feminine singular, the relative pronoun must match it, so Croatian uses kojoj.

So u mjeri u kojoj is literally in the measure in which.

Because the noun has a different role in each part of the sentence.

• In naručitelj radova, radova is in the genitive because it means of the works.
• In radovi su završeni, radovi is the subject of the clause, so it is in the nominative plural.

So the same noun appears in two different cases because it is doing two different jobs.

In Croatian, especially in construction, renovation, and legal/technical language, radovi very often means works, jobs, or construction work.

English often uses the singular uncountable work, but Croatian commonly uses the plural radovi for this kind of practical/project-based work.

So in this sentence, radovi is the normal and idiomatic choice.

Su završeni is a passive/resultative structure. It means that the works are completed or have been completed.

• su = are
• završeni = finished/completed

This focuses on the state of completion, not on who did the finishing.

By contrast, završili su is an active form meaning they finished. That would usually suggest that the subject actively finished something, which is not the idea here. In this sentence, the important point is that the works have reached a certain stage of completion.

It is a past passive participle, but it behaves a lot like an adjective.

That is why it agrees with radovi:

• radovi = masculine plural
• završeni = masculine plural

So grammatically it acts adjectivally, but in meaning it is part of the passive idea: the works are completed.

Yes. U mjeri u kojoj is quite formal, especially in legal or contractual language. In less formal Croatian, people might use something like onoliko koliko or another simpler structure, depending on the exact meaning.

But in this sentence, the formal wording is deliberate. It sounds appropriate for a contract, regulation, or official document.

Yes, Croatian word order is fairly flexible. But the original order sounds natural and appropriately formal.

For example, you could move the first clause for emphasis, but the given version is clear and neutral for legal style:

Naručitelj radova plaća u mjeri u kojoj su radovi završeni.

That order presents the subject first, then the obligation, then the condition/extent. This is very typical of formal written Croatian.