They do not all mean the same thing.

From bottom to top:

the planta cubierta is the "floor" the roof is on. If you look at these examples in Spanish:

http://homepages.mty.itesm.mx/al777807/mi_proyecto_en_autoca...

http://www.arquitectonia.com/viviboadilla/viviendaboadilla.h...

http://www.grupomirador.com/valle_planos.htm

what you will see is a plan/drawing of the roof itself and not the space underneath it. You can say "roof floor plan", see this example:

http://www.bbk.ac.uk/msp/fp_roof.html

However, as you can also see from the above example, the "roof floor" is actually the roof, and this is what we normally use in Engliah. So the above examples would more usually be termed "roof plans". Obviously you haven't given a lot of context. This could be referring to the roof plan, but if in doubt, play it safe and say roof.

desván.
(Del ant. desvanar, der. de vano).
1. m. Parte más alta de la casa, inmediatamente debajo del tejado, que suele destinarse a guardar objetos inútiles o en desuso. (DRAE)

Desvan is indeed the attic/loft as dicitonaries state. According to the penguin dicitonary of building, an attic is a habitable room in the roof space. And a loft is a store room in the roof space which can be converted into an attic.

Which brings me to the bajo cubierta (which is not in the DRAE as far as I can see). According to another dicitonary I have (LID construccion e inmobiliario) this is "definido por el espacio que media entre la cubierta y el ultimo forjado". Sounds pretty much like the roof space referred to above, described in Penguin as: cavity inside a roof, above the top-storey ceiling, either empty apart from services or containing an attic or loft.

HTH