1940's home additions and repairs

Hi, new to group so first post.
I bought my first home last year. It is a small two bedroom, two bath on a lot and 1/2 with a detached two car garage with an upstairs rental unit.
The house is about 1000 sq. ft. The living room is awkward for furniture, one bedroom is used for an office, and the other is used as master.
Off the kitchen there is a small room (about 8 x 8) that used to be either a mud room, a porch or a laundry room and the roof is gabled. It is enclosed and someone built a framed in space for the water heater (about 3 x 3). There's access from the kitchen to this space that leads to a door to the back patio and a door to the master bath.
The master bath was added next to the old mud room with two doors, the other being from the master bedroom. This door exits what I assume to be through the former siding of the house into this addition. The ceiling incurred some water damage after we bought the house and I had removed some of the ceiling drywall to ascertain the leak. The roof is flat pitched and meets the joists for the main house ceiling.
I would like to strip the master bathroom and start all over. The sub- floor under the cheap shower had rotted and he space is poorly used. I'd like to take the small door from the bedroom and make it into double French doors to a bathroom oasis and also add French doors to the outside back patio (that I might want to make into a room addition).
Some of the problems:
The mud room is concrete floor at a different level than the main house and the master bathroom. I've considered removing the 3x3 water- heater room and replacing the water heater with an "on demand" water heater. I've also considered framing in the door and making this into a Wash/Dryer area with storage. (Current washer/dryer is in garage about 35 feet from house.
Master Bathroom is poorly arranged, has paneling on the former siding, and needs to be stripped completely down to the studs.
I have a lot of time on my hands, plenty of carpentry, electrical, plumbing skills and tools that I could do all this work. My goal has been to design the bathroom first and then tear the old one out and begin building.
Does anyone have any advice or suggestions? I know for certain that if I didn't want to redo the bathroom, the sub floor MUST be replaced because the tile put in by the person who "flipped" this house is already cracking because of the water damage from the cheap shower. I want to frame a tile shower, add a tub, re-arrange the toilet, add ventilation, minimize the unnecessary linen closet and make this a very pleasant part of the master suite. I fear that if I do all this, the master bedroom may appear a joke by size comparison as the master bathroom would be almost 1/2 the size of the bedroom itself.
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residential designer, or maybe even an architect, if your budget allows. (If you live in an area that does not require an AIA stamp for residential work, there are likely a couple of quite competent designers around that charge less.) You seem to be focusing in on little details, when you need some trained and experienced eyes to look at the house as a whole, help you determine what you want to do with it, determine what will be resalable (if that is a consideration), what is even practical and/or code-compliant for your area, etc. Most designers do the initial walk-through meeting for free, or close to it, with costs after that depending on how much detail you want to get into. (From rough sketches, all the way to a set of working blueprints and engineering drawings with material takeoffs, ready to hand to city for permits and to contractor for estimates.) Fees vary greatly by area, but it may be the best money you spend on the place. Designing a remodel so it can be done in phases as budget allows is a concept they are quite familiar with.
Good example of why you should looked at flipped houses with a jaundiced eye. Sure, there are people out there who do it right. But there are a lot more who go in and do a superficial remodel/redecorate, don't really fix any of the underlying problems, and think the place is worth what a properly maintained and upgraded house is. A shack with new cabinets and carpet and fresh paint, is still a shack. They treat houses like used cars.
aem sends....
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