I come from a family with six boys and two girls. Some of my relatives had even bigger families. When it comes to a Catholic house, I think of a house with lots of kids. My ideal lots-of-kids home would include:
Bedrooms large enough that you have to share with one or two siblings. Three to a room is OK, but don’t go beyond four to a room. Do not skimp on storage; the camping gear and the hand-me-downs that aren’t needed yet have to go somewhere. People with money can buy as they go; frugal people who are lucky enough to have a piece of the earth want someplace to store the canning jars, gardening supplies, and so on.
Room to play: a play area that will take some punishment, where the noise won’t drive Mom nuts in bad weather. Consider 1/2 inch paint-grade plywood instead of drywall at least half way up the wall of this kind of room. A mudroom entry from the backyard and garage, for getting out of wet sloppy clothes and a place to hang coats to drip dry. A low-sided bathtub with a showerhead on a rope, for hosing the legs off. A big coat closet and room for sports gear. A place for a basketball hoop, for tricycles and bikes, and as much level grass for sports as you can manage. Kids are easier to discipline when they get plenty of time to run around. A fence and a tolerance for flowerbeds that have truck roads winding through them, not to mention the dog. If the play area for the smaller children can be kept under surveillance from the same area in which the parent is watching over homework, so much the better.
Kitchen with one double sink and one other sizable sink, laid out so that kids emptying the dishwasher or baking don’t get in the way of making dinner. (See if your library has “Kitchens for Cooks”) If the family is really big or you entertain the whole extended clan, think about two dishwashers, too. The most important communal space is a communal dining space. Everything else could be for smaller conversational groups, because that many people don’t sit and talk to each other one at a time very often! Besides, if you have a room that has a carpet, smaller kids sit on the floor.
Study area close to where study-time parent hangs out. Do not forget to have enough electrical outlets. At our house now, we have our kids’ computers, the TV, and the kitchen in the same room. I don’t know that this would work if we had more than two kids, but it would definitely be a good idea to be able to monitor all screen time until kids reach a certain age…and no screen time in bedrooms. Complete privacy and screen time need to be mutually exclusive. The dinner table can function as study table between meals.
If you have a piano, don’t put it in the same room with the TV. This can be in an area that is otherwise a quiet area, though.
Private parents room but not many steps to the baby’s room.
If you restrict the number of bathrooms, consider the Jack-n-Jill concept, where the person using the toilet and/or bathtub (shower) can lock themselves away from the person(s) using the sink and mirror. Otherwise, if you get even one primper, he/she will tie up the bathroom for too long. Make sure the locks in the Jack and Jill bathrooms cannot be used to trap a person in the bathroom, even if you have to have a door with two locks (one for each direction). (Did I mention I had six brothers?

) Unless you enjoy being late for Mass and listening to fights and excuses on the way to church on Sunday mornings, give every person over 12 access to a mirror. My sister swears by having one to look at right before you go into the garage to get into the car, too.
My parents had a pull-out stool under one of the bathroom sinks, so a little kid could wash their hands while standing on it. Great idea. I highly suggest faucets that can be turned on and off with an elbow, instead of the dual twist-ons faucets that look better.
Double the amount of immediate hot water capacity you think you need.
My mother had space in her laundry room for each person to have a folded-clothes cubby, plus a place for hanging clean clothes. She had another area for mending and always wished it was closer to her laundry. Each kid is responsible for putting his or her own clothes away.
Ample freezer space and somewhere to store home-canned food. Large families eat tremendous amounts of food.
Make it easy to clean and easy to touch up: spend the money for paint that you can scrub. Try to eliminate sunken rooms (stairs cause falls) and anything that can’t take abuse or would cause injury. Think in terms of ease of use, so kids can learn to help younger, and think in terms of being able to put gates in so the crawling one won’t hurt himself while you’re stuck in the bathroom doing toilet training. Think about putting the windows close to the floor, so the toddlers can see out without climbing. Think of built-in storage or how you will attach anything to the wall that could fall over. IOW, think of climbing toddlers, wrestling middle-schoolers, teens who don’t know their own strength, groups of children who
will use wheeled vehicles in your house when you told them not to, and build accordingly. It is money well-spent.