The smells you are used to living with will never bother you (hmmm -- except maybe in extreme circumstances when the odor sucks all the oxygen out of the room! Yes, I was in real estate and experienced that.) You may think your cats ALWAYS used the litter box but obviously, others know better.

Dogs aren't so bad because they don't spray the walls BUT they can also smell if you don't let them out and bathe them often enough. As far as wood floors go, you can't sand out cat urine. It will be there as long as the floor stays. Sorry --- you enjoyed your pets and this is the price. You will finally sell your house at a discount because you enjoyed those pets. Someone buying your house may have to replace drywall and floors and will expect you to pay for that. Sad but true.

I'd remove the sheetrock under the stairs completely, and redo it with new.

I'd also repaint all the walls in the house, if possible, on the off-chance that your cat has sprayed the walls anywhere. New paint smell is better than cat urine smell when it comes to selling. If your garage has drywall in it, I would paint in there as well, and possibly seal the floor.

Is any of your furniture still at the old house? I'd remove it completely in case it still smells of the pets.

Carmen - The wood floors were in a room we used all the time (daily). I would have noticed if the cat's peed on them. They didn't. These cats don't spray either.

Rivkadr - The house has been entirely repainted, recarpeted (we have carpet upstairs). The garage does not have drywall.

That's a bit of a stumper, then -- the feedback you're getting, is it saying that the smell is throughout the entire house, or only in specific areas (under the stairs, and in the garage)?

The feedback is that it is in the downstairs generally (not the upstairs) and not so much in the master bedroom. I think we are going to call in a professional odor removal service. Perhaps see if an ozone generator would help.

Yep, time to call in a pro. The house won't sell at a going price if it smells like cats. I have always felt that cats are really outdoor animals that might come indoors sometimes. Our experience is that the litter box is only used when it is below 0 outside and the snow is just too high for them to get out to do their stuff. (actually, they will dig holes in the snow if I let them out.) If they have their 'druthers', they want to eliminate outdoors far away from where they eat.

With our kittens, there is a litter box in the garage that I am confident the mom will teach them how to use. She will also teach them that the better alternative is to do it outdoors where it can fertilize the soil.

You might try painting the bottom of the stairs with a new coat of pain with an added splash some vanilla artificial flavoring in the paint can. Unfortunately cement flooring is very absorbent and odors can stay in the floors for years.

A coat of paint not pain....tho it will be a pain to do...

I watched TV before and I found some products to remove cat odors and then I buy one, but the odor is still there. I don't know what to do now.

Here is a link that might be useful: Cat Odor Removal

I can tell you that if you do have cat urine on carpet that you can pour a generous amount of club soda on it, then place a thick towel over the spot with some heavy books on it. This will draw out the odor and when you remove the towel and books just let it dry and then vacumn. It worked for me on a very nasty spot where my male cat urinated. Sorry I can't help with wood and carport floors, however, if the carport floor isn't painted you may try pouring club soda on it and letting it seep into the floor.