kono tatemono ha kaisya no honbu desu.

You’re right: normal Japanese is usually written without spaces.

This sentence is spaced out because it is probably aimed at learners, to make the parts easier to see:

• この = this
• 建物 = building
• は = topic particle
• 会社 = company
• の = linking/possessive particle
• 本部 = headquarters
• です = polite is / am / are

In natural Japanese writing, it would usually appear as:

この建物は会社の本部です。

A natural reading is:

この たてもの は かいしゃ の ほんぶ です。

In romaji:

Kono tatemono wa kaisha no honbu desu.

A few notes:

• 建物 = たてもの
• 会社 = かいしゃ
• 本部 = ほんぶ
• The particle は is written ha but pronounced wa here.

この means this, but it must be used before a noun.

So:

• この建物 = this building

This is different from:

• これ = this one / this thing

So you can say:

• この建物 = this building
• これは建物です = this is a building

A helpful pattern is:

• この + noun = this + noun
• その + noun = that + noun (near the listener)
• あの + noun = that + noun (over there)

This is one of the first things learners notice.

When は is used as the topic particle, it is pronounced わ (wa), even though it is still written は.

So in this sentence:

• 建物は is read tatemono wa

This is just a standard spelling rule in Japanese grammar.
The same thing happens with the particle へ, which is often pronounced e when used as a particle.

は marks the topic of the sentence.

So:

• この建物は means something like As for this building, ...

Then the rest of the sentence says something about that topic:

• 会社の本部です = it is the company’s headquarters

So the overall structure is:

• Topic: this building
• Comment about the topic: is the company headquarters

This topic-based structure is very common in Japanese.

Here, の links two nouns.

• 会社 = company
• 本部 = headquarters
• 会社の本部 = the company’s headquarters / company headquarters

A very common beginner explanation is that の often works like ’s or of in English.

So:

• 会社の本部 = the headquarters of the company
• or more naturally, the company’s headquarters

But の is broader than just possession. It can also show category, relationship, or description.

In Japanese, the describing noun comes before the main noun.

So:

• 会社の本部
  • 会社 describes what kind of headquarters
  • 本部 is the main noun

This means:

• company’s headquarters
• headquarters of the company

If you said 本部の会社, that would mean something more like:

• a headquarters-type company
• or a company of headquarters

—which is not what you want here.

A useful rule:

• In A の B, B is usually the main thing, and A describes or relates to it.

本部（ほんぶ） means headquarters or main office/base of operations.

It is not exactly the same as a general office.

For example:

• 会社 = company
• 事務所 = office
• 本部 = headquarters

So 会社の本部 specifically means the company’s headquarters, not just any office building.

Depending on context, 本社（ほんしゃ） is also common for a company’s head office or headquarters. Learners sometimes notice that both 本部 and 本社 can be translated as headquarters, but they are not always used in exactly the same way in every context.

です is the polite copula. In beginner terms, it often works like is / am / are.

So:

• 本部です = is headquarters / is the headquarters

In this sentence, です makes the statement polite and complete.

Important point: です is not always a perfect one-to-one match with English is. It is better to think of it as the polite way to say that something is something.

The plain form would be:

• この建物は会社の本部だ。

That sounds less polite and more casual/plain.

Japanese does not have articles like a, an, and the.

So Japanese often leaves that kind of information to context.

For example:

• 建物 could mean a building, the building, or just building, depending on the situation.
• 会社の本部 could be understood as the company’s headquarters from context.

In this sentence, English naturally uses the:

• This building is the company’s headquarters.

But Japanese does not need a separate word for the.

Sometimes you could, but the nuance changes.

• この建物は会社の本部です。
  = As for this building, it is the company’s headquarters.
  This is a neutral, natural statement about the topic.

• この建物が会社の本部です。
  = This building is the company’s headquarters.
  This can sound more like you are identifying it specifically, perhaps in contrast to other buildings.

So:

• は = marks the topic
• が = often marks the specific subject, especially when identifying or emphasizing

In many beginner situations, は is the more expected choice for a simple statement like this one.

Not exactly. A more literal breakdown is:

• この建物は = As for this building,
• 会社の本部です = it is the company’s headquarters

So Japanese is often organized as:

• Topic + comment

rather than strictly following English subject-verb-object patterns.

That is why a very literal gloss might sound slightly unnatural in English, even though the normal translation is simple and natural.

Sometimes, yes. Japanese often omits information that is clear from context.

For example, if everyone already knows which building you are talking about, you might hear something shorter like:

• 会社の本部です。 = It’s the company’s headquarters.

Or in casual speech:

• 会社の本部だ。

But in a textbook sentence, all the parts are included clearly:

• この建物は会社の本部です。

That makes it easier to understand the full grammar structure.