To get a 7yr / 1 ARM for 30 years … and I want to pay extra principal to pay it off in 6 or 7 yrs before it adjusts (presumably upward) … is generally allowed for an ARM (I know it's okay with fixed mortgage I am doing it now on my 15 yr fixed, effectively making it a 10 years old) … but we want to refinance and add a room so my question …. ARM has very low rates, which makes me wonder if they have a prepayment penalty clause.

make sure before you sign that you understand what the papers says. If you do not know, ask then for them to show you how it is in the documents.