Do i have to declare a 12hour job?

My fiance works full time and I am a stay at home mum to our one year old. we receive Tax Credits and also Housing benefit. I have an interview for a job..but its only a part time 12 hour job..am I right in saying I dont have to declare an under 16hour a week job? obviously I shall check with relevant professionals anyway...dont want to get done for benefit fraud by accident! - but thought Id ask quickly :) Roxy