It’s a Study. No, It’s a Guest Room.

Designing a guest room can present a conundrum: It’s nice to have a place to host overnight visitors, but a dedicated room uses precious space that sits empty most of the time. That’s why turning a guest room into a space that is used year-round — as a study, library, home office or den — can be so appealing.

Just ask David Mann, the founder of MR Architecture & Decor in New York, who routinely hosts friends in the study of the apartment he shares with his husband, Fritz Karch, in Manhattan’s Turtle Bay.

“I used to live in a studio, where everything had to be in one room,” he said. But even now that he lives in a much larger, 1,800-square-foot apartment, he dislikes the idea of space going unused, so he designed the study to double as a guest room.

“It’s filled with books, and we go in there to read or to have a quiet space” on a regular basis, he said. But when friends are coming, “we set it up in a very special way, a bit like a hotel room.”

Here’s how he does it.

Make the Bed

The most important furniture in a study that doubles as a guest room is a piece that provides a place to sit during the day and to sleep at night. The most common solution is to install a sleeper sofa with a mattress that unfolds when needed, but Mr. Mann prefers a simpler option: a daybed with a seat cushion the size of a twin mattress.