особняк in English:

They live in a mansion.

The cat burglar must have entered the mansion from the roof.

It is not a real mansion.

If my house were a mansion, I would invite everyone I know to my birthday party.

In addition to a mansion, he owns a Bentley.

A government official's stately mansion was looted.

He was very rich, and therefore could buy a mansion.

Keisuke has always studied in this mansion. That style known as 'home schooling', right?

There are no fewer than a dozen bedrooms in this mansion.

It was as gloomy inside the mansion as the sunlight was dazzling outside.