atarasii heya ha yatin dake de naku, kanrihi mo kakuninsita hou ga ii.

A natural way to break it up is:

• 新しい部屋は = as for a new room / when it comes to a new room
• 家賃だけでなく、管理費も = not only the rent, but also the management fee
• 確認したほうがいい = it’s better to check / you should check

So grammatically, the sentence is:

[Topic] + [not only A, but also B] + [advice expression]

は marks 新しい部屋 as the topic of the sentence.

It does not mean room is the subject doing the action. Instead, it sets the frame:

• As for a new room...
• When looking at a new room...

This is very common in Japanese. The sentence is giving advice about new rooms in general, so は is more natural than が here.

だけでなく means not only ... but also ...

It is part of the common pattern:

• Aだけでなく、Bも = not only A, but also B

In this sentence:

• 家賃だけでなく、管理費も
• not only the rent, but also the management fee

This pattern is very useful and common in both spoken and written Japanese.

A more casual version is:

• Aだけじゃなくて、Bも

So:

• 家賃だけじゃなくて、管理費も確認したほうがいい means basically the same thing, but sounds more conversational.

In the pattern Aだけでなく、Bも, the も means also / too.

So:

• 家賃だけでなく = not only rent
• 管理費も = management fee too / also the management fee

The も is what completes the not only A, but also B idea.

Without も, the sentence would sound incomplete or unnatural in this pattern.

This is one of the most common questions learners ask.

Here, 確認したほうがいい does not mean past time.
The pattern:

• Verb in た-form + ほうがいい

means:

• should do
• had better do
• it’s better to do

So:

• 確認したほうがいい = you should check

This is a fixed grammar pattern for advice.

Compare:

• 確認するほうがいい can sound more like a general comparison: it is better to check
• 確認したほうがいい is the standard, very common way to give advice: you should check

Literally, ほう can mean something like side or direction, but in this grammar pattern it is not translated literally.

In Verb-た + ほうがいい, it suggests a comparison between options:

• the option of checking vs.
• the option of not checking

So 確認したほうがいい means:

• the checking option is better
• more naturally: you should check

It is best learned as a set expression rather than word-by-word.

Because Japanese often does not mark the object with を when another particle is already attached and the meaning is clear from context.

Here:

• 家賃だけでなく
• 管理費も

are both understood as the things being checked because of the verb 確認する.

Especially with も, it is very common for it to take the place of を in practice.

So even though English clearly marks the object by word order, Japanese can leave that role to context and particle patterning.

In other sentences, you might see:

• 家賃を確認する = check the rent
• 管理費も確認する = also check the management fee

So nothing is missing in an unnatural way here.

Yes, the subject is omitted, and that is very normal in Japanese.

The sentence does not explicitly say:

• you should check
• one should check
• people should check

But from context, it is understood as general advice to the listener or to people in that situation.

Japanese often leaves out subjects when they are obvious.

So the implied meaning is something like:

• When looking at a new room, you should check not only the rent but also the management fee.

It is plain style, but not rude.

• 確認したほうがいい is a normal plain-style advice expression.
• だけでなく sounds a bit more neutral or written than だけじゃなくて.

If you want to make it more polite, you could say:

• 新しい部屋は、家賃だけでなく、管理費も確認したほうがいいです。

That sounds more polite in everyday conversation.

A more casual version would be:

• 新しい部屋は、家賃だけじゃなくて、管理費も確認したほうがいい。

It can feel a little broad, and in this sentence it is most naturally understood as a general topic:

• when it comes to a new room
• for a new apartment/room you’re considering

Because は marks a topic, the phrase is less about one specific room doing something, and more about the case of a new room.

If the speaker wanted to be very specific about one particular room, context would usually make that clear.

So in this kind of advice sentence, 新しい部屋は often has a general, category-like feel.