When I read your description and the fact that a bathroom was added to the basement, makes me think you are smelling sewer gas. Is there a plumbing trap connected to the tub drain? If not then that could be the source of the smell. Fill the tub with some standing water and then open the deck door. Determine if there if the smell still exists. Repeat this with the bathroom sink. If this is not it, then I am also baffled.

If you have leaky return air duct work the basement,its sucking basement air into your home upstairs. What do the walls look like? can you seal/close the windows? Any bulk, liquid water, draining from your a/c or dehumidifer, needs to be draining into a covered drain. Insure your basement is seal as much as possible. Stop the mirgration of vapor into your basement 1st. And your stop the mold growth. If your have ground..it must be covered. if you have concrete, with lots of cracks, seal them...U may consider sealing the concrete floor...but this isnt the place for anymore of this information. Get a pro involved.

this smell is most likely a type of mold, I just bought a house built 1906 with a very tight crawl space strong amonia smell and noticed a black mold on some spots under the house some that actually loked like black hair growing like grass , but there is also a openin g in the sewer line but in my experience with sewer gas it rarely smells like mold either way you need to take this seriously because mold can make you seriously ill (has been linked to some cancers) and methane gas produced by the break down of sewage is hazardous, not only that it makes you sick and smells bad but its highly explosive houses have been completely destroyed from explosions caused by methane gas ,I am planning on mounting 2 salvaged furnace blowers under thge house directly to the foundation vents these will pull the air from the vents on the opposite side thru drying and airing out the crawl space, you can go one step beyond and wire these thru a humidistat so that they come on when the humidity/moisture reaches a ceartain level , doing this will eliminate the mold becdause mold can not survive in dry climates BUT MAKE SURE ITS NOT A SEWER LEAK BECAUSE THE SWITCHING ON AND OFF OF THE BLOWERS CREATE A SPARK THAT COULD IGNITER THE METHANE IF ITS UNDER THERE. by the sewer she lived... by the sewer she died ...they said it was murder but it was......... sewerside

I've been meaning to post about this very thing myself. We bought a dresser a couple of years ago to put family pics on top of, and I had planned to store dining room linens in it, but we've never completed gotten the stink out of the drawers. We tried coffee, baking soda, and a heavy-duty tub of air freshener (from Home Depot or Lowe's), and the smell is still not completely gone. We also gave all the drawers a good cleaning with Scott's Liquid Gold. We not-so-affectionately call the piece "Old Stinky." I've thought of resorting to priming and painting the insides of the drawers.