"우리는 시외에 집이 있었어요."
Translation:We had a house outside the city.
One is native Korean, the other is Sino-Korean. Sino-Korean words are used with other Sino-Korean words
although Matthew is right, remember that 시외 is a singular word and it's best translation would be 'suburbs'
also don't try to randomly use '외에' the same way as '밖에'. Although I am not 100% sure, '외에' by itself doesn't really mean anything nor is ever used