Hi @Judith870
I think that will take you from 4 to 5 guests?
You could search around your area to see whether 5-guest listings get significantly better prices. Keep in mind:
1) One extra guest won't necessarily increase costs very much (amenities, etc.), but more people do make for more cleaning and (especially) laundry.
2) You probably can't make the sofa bed for them - you need enough easily accessible cupboard space to leave all the required bedding for the guests.
3) You risk taking a hit in reviews (which are presently great) if your guests feel overcrowded with 5 guests and just 1.5 bathrooms.
If you do decide it's worth it to add a sofa bed, perhaps limit it to a single bed and advertise it as a place for a child to sleep?
Hope it helps.