Il y a means both "there is" and "there are" - it never changes for a plural. In a question you can simply raise your voice (Il y a un jardin ?), and in the negative it becomes il n'y a pas de, with de replacing un, une and des.
Examples
- Il y a trois pièces.
- There are three rooms.
- Il y a une salle de bains en haut.
- There's a bathroom upstairs.
- Il n'y a pas de jardin.
- There isn't a garden.
The mistake to avoid
Never "il y sont" for a plural: il y a covers one thing and many.
Where this fits in
Home & neighbourhood, French A1: Describe your home and its rooms, say where things are, and talk about the places near where you live.