צריך לקבל "האם יש לכם עליית גג? "
I had the same question.
Yesh lahem aliyat gag?
האם יש לך עליית גג isn’t acceptable?
Can I just say עלייה?
Not really. Maybe when it is very, very clear you are talking about an attic. But, when there is not context, you need to say עליית גג, otherwise when you say just עלייה, the first thing that comes to mind is "aliyah" or "increase".
Can I just say יש לך גג
No. גג on its own is roof.