The paths that sewer gas can may enter a bathroom is through the drains of the toilet, lavatory, or bath/shower. I would not suspect the toilet since water traps are built-in to that appliance. Water traps for the lavatory and tub or shower drain are constructed by the plumbing. Look under the sink to see if it has a trap. It will be difficult to see under the tub or shower floor, but look down the drains of these for standing water. These traps are standard and building inspectors should have looked for these, but these are so common those could have been over-looked. The other possibility is faulty vents or lack of a vent. The bathroom drains should be vented to the outside of the house, usually up the inside of a wall and through the roof. A differential pressure can develop between the street sewer and outer space. One source is wind. If the vent is plugged or improperly installed, wind gusts can slush the water back and forth in the traps and flush out enough water to break the water seal. After that, sewer gas can enter the room. If your sense of smell is keen enough, you may be able to tell by which source the gas is entering the room. By the way, sewer gas is combustible. It can be ignited if its concentration becomes great enough.

OP already has many Ideabooks. @Laurie Jarolimek (Teacher - Grade Pre-Kindergar, if you would like help with tasks still remaining before you, say "should I use blue towels?"; or decisions you're wondering if you should revisit, such as, "should I change the swing direction of this door?", all of us will happily chime in. Some people who've gotten feedback from Houzz on an aptly named "Design Dillema" post their final photos so Houzzers can see how it all turned out i.e. "Here are the blue towels you all recommended!" There is a Building A Home thread, restarted every month, for people actively involved in building a home. Here is this month's: https://www.houzz.com/discussions/6214319/january-2022-building-a-home#n=269

11'x11' is a generous space for a bathroom. You could, if you wished, fit 2 bathrooms in that space and still have a reasonable amount of room. For example, a common bathroom size is 5' across and 8-12' long. For both convenience, future children and resale reasons, I'd strongly recommend having a separate en suite since you have space for it.

When it comes to wet rooms waterproofing and proper pitch to the drain is essential. Based on our experience wet rooms or showers will get water under tiles trough fissures or any hairline cracks on grout and saturate the base, but will not cause leaks only damages to the setting base or loosen, stain and crack tiles with time. All wet rooms are not water resistant at the joints, they will develop fissures or cracks as the structure settles, topical sealers alone will not prevent water infiltration. We recommend to properly repair the grout as soon as it develops fissures to prevent further damage to the materials, the idea is to make all the moving parts/joints water resistant at which point sealer applications can work more efficiently.