Ortak giderin artması durumunda kat malikleri kurulu yeniden toplanacak.

Because giderin is in the genitive case.

In Turkish, a very common pattern is:

X-in Y-ması = the Y-ing of X

So:

• gider = expense / cost
• giderin = of the expense
• artması = its increasing / the increase

Together, giderin artması means the expense increasing or more literally the increase of the expense.

This is why Turkish does not use a structure like English if the common expense increases here. Instead, it turns the whole idea into a noun-like phrase.

Artması is a nominalized form of the verb artmak.

• artmak = to increase
• artma = increasing / increase
• artması = its increasing

The ending -ma/-me turns the verb into a noun-like form, and the -sı/-si/-su/-sü part is the 3rd-person possessive ending that often appears in this structure.

So:

• artmak = to increase
• giderin artması = the expense’s increasing

Turkish uses this kind of nominalized structure very often before words like durumunda, halinde, nedeniyle, and so on.

Durumunda means something like in the case of, in the event of, or simply if in a formal style.

It comes from:

• durum = situation / case
• durumunda = in its case / in the case of

So:

giderin artması durumunda
literally = in the case of the expense increasing

Natural English:

• if the common expense increases
• in the event of an increase in common expenses

This is a very common formal Turkish pattern:

• ... durumunda = if / in case / in the event that

Turkish can do both.

This sentence uses a more formal, administrative, written style.
Instead of a simpler conditional verb form, it uses a noun phrase:

• ortak giderin artması durumunda

A more conversational alternative could be:

• Ortak gider artarsa, kat malikleri kurulu yeniden toplanacak.

That also means If the common expense increases, the board of owners will meet again.

The difference is mainly style:

• artarsa = more direct, more everyday
• artması durumunda = more formal, official, bureaucratic

It is a set expression meaning the board/council/committee of unit owners / apartment owners / condominium owners.

Breakdown:

• kat = floor, storey, unit; in property language it is part of the legal term
• malik = owner
• malikleri = owners
• kurul = board / council / committee
• kurulu = its board / the board of ...

So kat malikleri kurulu is a noun compound meaning something like:

the board of the apartment-unit owners

In practice, it is a legal or administrative term, so it is best learned as a whole phrase.

Because this is part of a Turkish noun compound.

In Turkish, when one noun is defined by another noun, the second noun often takes a possessive ending:

• öğrenci konseyi = student council
• şirket merkezi = company headquarters
• kat malikleri kurulu = board of unit owners

So here:

• kurul = board
• kurulu = its board / the board of ...

That -u does not mean a separate word like English its in a simple sense; it is just how Turkish builds this kind of compound.

Because the true grammatical subject of the sentence is kurul, not malikleri.

The subject is the whole phrase:

kat malikleri kurulu = the board of owners

That whole phrase refers to one board, so the verb is singular:

• kurul ... toplanacak = the board will meet

Even though malikleri is plural, it is only part of the larger noun phrase describing the board.

Not in the way you would normally understand English passives.

It comes from the verb toplanmak, which means:

• to gather
• to assemble
• to meet
• to convene

So:

• toplanacak = will gather / will meet / will convene

It may look related to toplamak = to collect, but toplanmak is a standard dictionary verb in its own right. In this sentence, it simply means that the board will hold another meeting.

It is the future tense.

• toplan- = verb stem
• -acak = future marker

So toplanacak means:

• will meet
• will convene
• is going to meet

Because the subject is third-person singular, Turkish does not need an extra ending here.

Yeniden means again, anew, or once more.

In this sentence:

• yeniden toplanacak = will meet again

Yes, tekrar could also often work, but there is a slight nuance:

• yeniden can sound a bit more formal or like anew
• tekrar is a very common everyday word for again

In an official or administrative sentence like this, yeniden sounds very natural.

Yes. Even though gider is singular in form, Turkish often uses singular nouns in a general or collective sense.

So ortak gider can refer to:

• the common expense charge
• the shared maintenance cost
• common expenses in general, depending on context

In English, translators often choose common expenses because that sounds more natural in housing/management contexts. But Turkish does not always need the plural form where English prefers it.

Yes. Turkish word order is flexible, although the verb usually stays at the end.

This sentence starts with the condition:

Ortak giderin artması durumunda ...

That gives the sentence a formal, topic-first structure.

You could also say:

Kat malikleri kurulu ortak giderin artması durumunda yeniden toplanacak.

That is still correct. The difference is mostly emphasis:

• starting with Ortak giderin artması durumunda emphasizes the condition
• starting with Kat malikleri kurulu emphasizes who will meet

The original version sounds very natural in formal written Turkish.