Alternative venting for bathroom extractor fan?

Hi all! We are intending to install a shower room into a vacant cubby-hole. Everything fits, except the vent for the extractor. There is only one exterior wall and that has a cement lintel at vent height, with a window below. There is not enough of a gap above the lintel and below the joists, not enough space to the side without going through the party wall or through a narrow piller which would then need steel support. The run through other rooms would be unsightly and I am guessing, inefective. Will we have to fit a smaller window, or is there some other alternative? Many thanks in advance!