Das Haus nebenan ist unseres.

Nebenan means next door or in the neighboring place/building.

So das Haus nebenan means the house next door.

You can also use nebenan on its own:

• Sie wohnen nebenan. = They live next door.

Because das Haus nebenan is a normal German way to say the house next door.

Even though nebenan is an adverb, German often uses it after a noun like this in fixed, natural expressions:

• die Leute oben = the people upstairs
• das Café gegenüber = the café opposite
• das Haus nebenan = the house next door

You would not normally say nebenan das Haus here.

Because unseres is standing on its own and replaces the noun phrase unser Haus.

Compare:

• Das ist unser Haus. → unser comes before Haus
• Das Haus ist unseres. → unseres stands alone and means ours

In other words:

• unser Haus = our house
• unseres = ours

Since it refers to das Haus, which is neuter singular, the correct form is unseres.

It is nominative neuter singular.

Why? Because the verb sein is a linking verb. It does not take a direct object here. Instead, it links the subject with a complement.

So in:

• Das Haus nebenan ist unseres.

Das Haus nebenan is the subject, and unseres refers back to it. Because it matches das Haus, it is also nominative.

The -es ending may look like genitive, but here it is not genitive.

Because unseres already functions as the whole noun phrase by itself.

Just like in English you say:

• It is ours.

and not:

• It is the ours.

German works the same way here:

• Das Haus ist unseres.

not:

• Das Haus ist das unseres.

Yes. It is a very natural alternative.

• Das Haus nebenan ist unseres. = The house next door is ours.
• Das Haus nebenan gehört uns. = The house next door belongs to us.

Both express ownership. The second version uses gehören + dative, so you get uns.

For many learners, gehören is an easy and common way to talk about possession.

They are not the same.

• nebenan = next door
• daneben = next to it / beside it

So:

• Das Haus nebenan = the house next door
• Das Haus daneben would usually mean the house next to some previously mentioned thing

For a neighboring house, nebenan is the usual choice.

The possessive form changes to match the noun it refers to.

Examples:

• Der Wagen nebenan ist unserer.
• Die Wohnung nebenan ist unsere.
• Das Haus nebenan ist unseres.
• Die Häuser nebenan sind unsere.

So the form of our/ours depends on the gender and number of the noun being replaced.

Yes. In casual speech, you may hear unsers.

So people may say:

• Das Haus nebenan ist unsers.

But unseres is the full standard form and is safer for learners, especially in writing.