I think it would be better to put it like this, since we cannot be absolutely certain that there are just two storeys; it is very likely that the "altos" are just one upper floor, but it is not impossible that there are two upper floors (three floors in all). "Upper and lower floors" covers all possibilities, since it doesn't specify the number of upper floors.

"Altos y bajos" in house descriptions normally refers to the kitchen cupboards, but surely not here. It is being used as in the following example:

"¡¡¡oportunidad¡¡¡¡ casa (altos y bajos )totalmente nueva con cochera ,en el centro."
http://www.spainhouses.net/es/merida/comprar/casas/casa-vent...

The accompanying photos show the staircase and make it clear that it refers to a house with an upper floor or floors.

"Granthams Landing, 481 Central, 3 Story "Main level entry with upper and lower floors". Residential"
http://joevallee.point2agent.com/Property_Listings/page_5698...