Personally I would choose to add a 4th bedroom and bathroom via an addition rather than trying to make 4 bedrooms out of the existing 3, if even possible. You can have an architect work up both options and have them priced out by a contractor.

Those bedrooms are not overly large now. Do you want to have a bed larger than Full in any of them?

The only thing i can see would be to change the storage to a tiny three piece bath with a door into the smallest bedroom to have an ensuite.

Mystery meats

We don't see the "extension"

We don't see the second floor, IF there is a second floor.

We don;'t know how many in the family, or for whom the extra beds shall be, their gender, who can share etc

We don't know enough about any if it and you need more information in your post.

That storage area could be another bath, but beyond that and better use of the space?

You need a lot more information : )

" we are already doing a rear extension on the first floor "

Larger rear addition maybe? Without real context it is impossible to really answer your question. Throw enough money at it and anything is possible.

THose bedroom spaces will not be able to give up space to make one more bedroom and really that storage is not going to reall provide a decent bath either so just do a 2nd floor extension for what you want is probably the answer. We need a ton more info to have any idea really.

The bedrooms are a decent size, but definitely not "very large", specially if you have 2 kids sharing a bedroom.

Is this an older house? If so, some of those are bearing walls. You might find a renovation to achieve your goals is more expensive than an extension, especially since you are already planning one.

thanks for the questions and helpful input. I grew up with a family of 4 in a 850sf apartment so to me these bedrooms feel cavernous.

budget certainly matters, thus why i pose this question to see if i could get away without extending two floors to the rear. I guess it might be best to add to the second floor as well.

And yes , this is the second floor of a turn of the century kit home and the rear extension would most likely be 10—14 feet out, square to the house.

thanks again! perhaps ill get your input on how to configure the new kitchen /dining for best function and flow!

These are average size bedrooms and the overall footprint here on this upper floor is not large enough for an additional bedroom and bathroom. Where is you lower floor addition being added onto in reference to this upper floor? It is less pricey to build up than build out , but now is the time to make this choice, not go back and add the second floor addition later. Show us the overall downstairs floor plan and where the addition is being added and then we can help you plan the best line of action for the upper floor. The two need to function with each other.

Another option may be to expand the 1st floor addition to include the 4th bedroom & bath on the main level. It can be very handy to have a main floor bedroom and full bath if, for example, you plan to age in place, are recovering from a surgery or will house an aging parent.

The most expensive addition will be to the main floor since you need foundation, so you may be pleasantly surprised with the relatively “inexpensive “ cost of adding to the second floor while you’re at it. A big, but exciting project!

Here is the first floor - previous owners enclosed a small back porch to become a kitchen. its poorly functional and every gets stuck in the kitchen like a dead end.

plan is to chop off the existing kitchen, extend directly out , square to the original house. there is no plan yet but wants are adding mudroom, pantry, breakfast nook, and an exit to the rear for more inside outside life. major need is function and flow. We host 20 -30 people often and would like to up that number.

the office is used as a playroom atm, which will become a work space or art studio as they age. we use the side door to enter. thanks

So essentially you have the perfect opportunity to build a owners suite and possibly even upstairs laundry , depending on how large the add on downstairs is, above that add on. You will have the perfect access point by eliminating /relocating the upstairs storage closet. Plus you have the upstairs bath on that outside wall to tie your new ensuite and laundry room plumbing into.

I'm not seeing a foursquare layout here. I lived in one for 20 years. It is a cube with four bedrooms, and a central bathroom. Typically, there is another staircase from the central second floor landing that goes up to the attic. Because of the shape of the roof, you would add a dormer to get another room. With some professional design you could get a bathroom in that area, over the second floor bathroom, with some kind of a skylight.

If it were mine, $$ no object . . .

I closed off the front entry you said you don't use, and eliminated the awkward "step up from two sides" approach to the second floor. (That does make kids go around through the living room to reach the half bath, though, so you'd have to decide whether you'd rather keep it.) Kept plumbing as close as possible to existing. Tried to make a more welcoming side entry, adding a window and opening the wall at the top of the stairs a bit. Minimal pantry (two cabinets flanking approach to kitchen) and mudroom (two coat closets between living and dining), though. Decent flow for entertaining a group, perhaps?