En primera línea de playa

En primera línea de playa
 

how can we say tengo un piso en primera linea de playa?

I have a beachfront apartment.

Here, you would also say I have a frontline apartment. This doesn't specify whether there is a beach in front of you, or just clifftop. The important issue is that nobody can build anything in front of you, so your sea views are guaranteed.