У будинку дев’яносто вікон.

Because after у meaning in, Ukrainian uses the locative case.

• dictionary form: будинок = house
• after у/в: у будинку = in the house

So будинку is the locative singular form of будинок.

Because after дев’яносто (ninety), the noun goes into the genitive plural.

For вікно (window), the counting forms are:

• 1 → вікно
• 2, 3, 4 → вікна
• 5 and higher → вікон

So:

• одне вікно
• два вікна
• п’ять вікон
• дев’яносто вікон

That is why вікон is correct here.

It is the genitive plural of вікно.

This is a very common pattern in Ukrainian after numbers like 5, 6, 7... 90, 100, etc. You also see the same form after words like багато (many/a lot of):

• багато вікон
• дев’яносто вікон

So even though the sentence is about counting, the noun itself is in the genitive plural.

Yes, in a way.

Ukrainian often omits є (is/are) in present-tense sentences like this. So:

• У будинку дев’яносто вікон.
• У будинку є дев’яносто вікон.

Both are possible.

The version without є is very natural and common. Ukrainian often leaves out the present-tense to be where English requires is/are.

У and в both can mean in. Ukrainian often switches between them for euphony—to make pronunciation smoother.

Here, у будинку sounds easier than в будинку, because в будинку would put two consonants together awkwardly at the start.

So this is mostly a pronunciation/sound-flow choice, not a meaning difference.

The apostrophe shows that the consonant before it stays hard, and that я is pronounced as a separate ya sound.

So дев’яносто is pronounced roughly like:

de-vya-NO-sto

The apostrophe prevents the в from simply being softened. Instead, you hear a clearer v + ya sequence.

This same idea appears in many Ukrainian words with я, ю, є, ї after certain consonants.

Ukrainian word order is more flexible than English word order.

Here, putting У будинку first sets the scene first: in the house... Then the sentence gives the information about what is there: ninety windows.

This order is very natural for an existential statement:

• У будинку дев’яносто вікон. = In the house, there are ninety windows.

You could change the order, but the emphasis would change. For example:

• Дев’яносто вікон у будинку sounds more marked and less neutral.

So the given order is a normal, natural way to present the information.

No. Ukrainian does not use articles.

So будинок / будинку can mean a house / the house, depending on context. English has to choose a or the, but Ukrainian usually leaves that to the situation.

That is why У будинку дев’яносто вікон can be understood as either:

• There are ninety windows in the house
• There are ninety windows in a house

depending on the broader context.

The basic pattern is:

• 1 → noun like singular: одне вікно
• 2, 3, 4 → special plural form: два/три/чотири вікна
• 5 and higher → genitive plural: п’ять вікон, дев’яносто вікон

A useful extra detail: numbers ending in 1, 2, 3, 4 do not always follow that pattern if they are in the teens.

For example:

• 21 вікно? No → двадцять одне вікно
• 22 вікна
• 25 вікон
• but 11, 12, 13, 14 all use вікон

So:

• одинадцять вікон
• дванадцять вікон
• чотирнадцять вікон

In your sentence, дев’яносто clearly belongs to the 5-and-higher type, so вікон is required.