Can it be vented through the exterior wall? We have a fan in our bathroom that vents out through a brick wall.

no, as it is a unit we can't get permission to vent through to exterior. Any ideas?

The second builder sounds correct -- if there is no escape route for all the moist air, it just stays there. Is it legal where you live to build such a bathroom? With no ventilation and no fan ?

the bathroom is existing in an old block of units- this is a renovation. It has a window about a metre wide so there is ventilation , but we can't fix an exhaust fan into the glass. If we have a fan in the ceiling I still dont see how we could "connect"that space to the window below it.

I understand the constraints in an apartment block it is similar - external vents are not permitted in existing builds if they weren't there in the first place). (In the UK)

In this set up, you are expected to open your window after your shower/bath and then close it when it gets cold.

For your drier (the biggest source of humidity) a solution could be to use a flexitube to the window, ie, you create the venting and place it by the open window when you are using it, and then remove it when the drier is not in use.

Gas or electric dryer?

Thank you for the tip on the dryer- only problem is the unit is tenanted and whether this will be done will depend on the whim of the tenant. The tenant supplies their own dryer- they are electric.
Dropping the ceiling 30cms to put in the 4-in-1 is a big sacrifice if it not likely to work well. Does anyone know of any other fan or heat type device that can be used?? Or maybe i just need to jam open the window!