Dun Laoghaire-Rathdown councillors have voted to prevent student accommodation being used for other purposes "while demand remains high".
Planning authorities must now be satisfied that there are "compelling grounds" in order for the purpose of accommodation to be changed.
The motion was proposed by Glencullen-Sandyford councillor Oisin O'Connor.
It doesn't affect the accommodation being used for different purposes during the summer months.
Cllr O'Connor said at a council meeting on Monday: "We have a high demand for student accommodation in the county. It takes a long time for the student accommodation to be built.
"Where permission is granted and student accommodation is built, it would be sensible for that particular building to continue as student accommodation".
The motion was passed unanimously.
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