How soon are you planning to have a baby? How old is your grandfather? Are your and/or DH's parents still living? Might they need to move in at some point in the future? If it was me, I would plan for both or know that the potential is there to lose your study to a bedroom if both a baby & a relative need rooms.

In addition to those questions already asked...
How old is the step daughter and how often does she stay with you?

My grandfather is in his 70s. I dont think he will be around for that much longer, but I dont want to be in a house situation where I am sort of waiting for him to pass.
My DH parents are still very much alive and I am pretty sure they have plans for themselves when they get up in age. As far as my parents go...I love them, but will not allow them to move in with me regardless. Long story that I would rather not get into.
My step daughter is 15. We will not be building the house until next spring so she will only be in the house for about a year before she goes to college. I know it makes sense to use her bedroom for other uses, but what happens when she comes home for the holidays and summer vacation?
As far as a baby goes we are not sure if we are going to have one or not. We have been back and forth about it the past couple months.
I feel like we are stuck. If we choose not to have babies in the future and built a larger house than we need we will regret it. If we dont build for it and decide to have a child then we will regret not building with enough room
Giving up the study is really a non option. There must be an office.

On the 'social' side of this...

A man in his seventies could live another twenty years. Is living in your home his only option? Could he live NEAR you? We are assuming he doesn't want to stay where he and his late wife were living. (Sounds like a retirement home; not somewhere they felt established.) Does he have family other than you in your area?

Your stepdaughter could need to live with you after college, and she should always feel she has a place in your home until she establishes her own home.

Whether your DH wants to father babies after a 15 year hiatus is between the two of you! LOL Should you have more than one, you'd probably need another house.

Must you build a new house now, with so much unsettled?

Is there the potential to add a bedroom/bathroom in the basement if needed in the future? Are you building a ranch? Is there a bedroom on the main level? If not, will your grandfather be able to manage stairs at 70? Even if he can now, he may not be able to in the future.

I think you are okay with 3 bedrooms, but it would be nice to have the potential for growth either in the basement or an attic conversion or bonus space over the garage.

I think what separates good design, from great design, is building flexibility into your home plan.

Try to include spaces that can change as your wants/needs change. If the bedrooms are upstairs, make sure you have a spot for a full bathroom and bedroom downstairs, for aging parents...or yourself, someday. For now, it can be a guest room/den.

If you're not sure how many bedrooms you need...then plan for extra spaces, usually called bonus rooms or even flex spaces...to use, at your discretion. A bonus room can be divided in half, to be office and guest room for step-daughter (as she goes to college) and her room could be used for the baby. Or the bonus space would make a lovely nursery/office for you (when you want to keep an eye on the baby) and a wonderful bedroom/playroom for the baby, as he/she grows older. If you don't need all the bedrooms, at some point it can be a rec room, for baby, as he/she becomes teenager :)

The same flexibility should be considered, when designing your main living area, too. Will your kitchen layout accomodate grandpa and/or baby? Will you have room for the high chair and play pen? Will grandpa be able to use the kitchen, easily? Do you plan to have lever switches throughout home? Are you considering lighting and dark/light contrast throughout your home (such as at the bottom of steps or between countertops and floor? This makes it easier to judge where one surface ends and another begins, especially when our eyesight isn't what it once was.

Hope this gives you some good ideas...and be sure to check out universal access, when designing your home. You want your home to be everything it can possibly be, rather than just the average home. That's what custom build is all about, right? :)

Actually the home he is in is his family's home. As in his father actually built that house. He has lived there for a loooong time. All his relatives are buried on the property. He has some acreage. When my grandparents were younger they had a Christmas tree farm on their property. I would never dream of asking him to sell it. It doesn't cost him anything but about $70 in taxes. He doesn't want to leave, I want him to. He fell a little while back. He said that he laid in the woods for close to three hours before he could get up! Thats why I want him to move.
My maternal family is very small. He had four brothers and sisters, but one is in the other side of the country another is nearly gone and the rest are gone. My mother is an only child so we are all he has.
Thank you for all the ideas to think about. The master bedroom and the study are both on the ground floor then there are two bedrooms upstairs. The idea about space above the garage is an excellent idea. I do always want my step daughter to have her room. Once she goes to college maybe she would appreciate being above the garage, ya know having some private space.
Surely finishing the space above the garage would give me enough room. I do have a lot of other things to take into consideration.

I disagree with the idea that a custom home needs to be everything it possibly can be--everyone works under constraints--budget and lot size being two of the biggest.

Everybody makes tradeoffs--and if something happens that makes them wish they'd chosen differently, then they go from there.

For example, we only have one bedroom/bath on the main floor. If one of the kids breaks a leg and can't handle the stairs, we'll have to adjust for that. But we didn't build a second bedroom/bath on the main floor "just in case".

Some people plan want a whirlpool tub in the master bath. We didn't want one and chose not to allocate the floor space to it. That means there isn't a bathtub on the main floor. A future buyer might have a problem with that, but if that's a deal breaker, it's a risk we chose to take.

One of us may have mobility issues at some point, requiring a wheelchair. We didn't build for that. If it happens, the access to our bedroom will need to be changed as well as the master bath. Another risk we chose to take.

Speaking as a mom and stepmom, I'd want to keep my child's room as her room until she could reasonably be expected to be on her own--in the case of a college bound 15 year old, that's at least 7 years. My college kids' rooms are mostly guest rooms now--but I haven't redecorated (or renamed them).

It sounds to me as if the OP has so many open scenarios that it makes it REALLY hard to plan. My first inclination would be to postpone the decision until you decide if you're going to have a child--that would make a big difference in plans.

If you can't really decide, my second inclination would be to plan a 5 bedroom house, although one bedroom could be smaller or non-optimal. (Master, step-daughter, grandpa, baby, guest). It might be possible to build a 4 bedroom house with a basement, knowing that if things change, you can finish the basement (or bonus space above the garage, as lavender suggested).

No house is going to be all things to a family for all time. That's just the way things go. You make your best guesses and you balance the risk factors, and go from there.

I agree with lavenderlass as to building a home that has flexibility in it. The home we are almost finished building has been laid out so that the rooms flow with minimal choppiness to the house and huge poetential for future needs.

We have an open floor plan that I love, but is set up so another room could be built within if ever desired....but currently absolutely no desire to close in my house. My husband and I enjoy being together, so I set up an open section to have as my sewing space rather than making a closed off room.

We put in a 4 foot wide hall to the bedroom and 3 foot doors. We made all the doors high quality pocket doors (ball bearing 300 lb.capacity Johnson hardware and custom solid wood doors). Nothing is worse than a poor quality pocket door set up. We have no doors to get in the way and research shows that pocket doors are easier for handicapped people, in most cases. The shower is 6 ft by 4 ft. and I gave up having a bathtub I would probably hardly ever use in order to have it. All the sliding glass doors are 8 ft. wide rather than 6.

The bathrooms have a nice layout that allows for wheelchair space to turn around in.

Our planning was not only if we become ill and handicapped, but also a car, skiing, horseback riding, or hiking accident can also put one of us in a wheelchair.

We built the basic structure for the "what ifs" since we want to age in place. It doesn't look like a handicap home because we are not currently handicapped, but can be made such with minor remodeling in the future if needed, such as a ramp rather than steps to our porch, a kitchen layout that allows for wheelchair friendly cabinetry to be installed....things like that.

I would be hesitant to build a house that has a bunch of small rooms just for the sake of adding bedrooms. Once the kids are gone they usually become empty useless areas that still need to be cleaned, with the likelihood they wouldn't be the size or configuration desired for something else.

We will make do in our current house when family comes to visit. I wanted a house that suits our day to day life, not for the few times a year that something else is "needed". Every space in a house should be used or it is a waste of money in terms of upkeep, climate controlling, and taxes. We actually have a blast when everyone is "camped out".

Take as much time as you need to carefully plan your home the way you want it. Seldom can we have everything in a home that we want, but we can come very close. Children can share bedrooms. We had 5 children and they loved being together. Boys in one large bedroom, and girls in another. They shared a bathroom (not at the same time:) They learned about sharing, tolerance, and how to compromise and get along, something that is sorely lacking in society these days. Nothing wrong with that, and we had no problem selling that home. I think it is silly to plan a home around "future buyers". A home should be your safe haven that makes you happy to be there, and well.... a home. Not like something sitting on a store shelf waiting to be sold.

While I appreciate everyones input I dont think I have come any closer to an answer. We did get my grandfather to come for the holidays...6 hour drive to get him. I was saddened by the space that he is living in. He has 3 dogs that have over run the house. There is a strong foul odor. He refuses to let anyone come in and clean. He coughs really badly. Im sure it is from breathing in the urine and feces odor. The dogs are well cared for, dont think that it is a cruelty situation its just that they mess on the carpets and he doesnt see it as much as a younger person would. My grandmother took care of him for so many years that he doesnt get that you have to mop a floor and wipe walls. He still has my deceased grandmothers hospital bed in the middle of the living room! He isnt moving on with his life, just stagnating. When we took him back I was praying that he would call right after we left and ask us to come get him for good.
I asked if he would come live with me. He said that he is not even entertaining the idea at this point. Im so upset about the whole situation. We were talking about it and we (the family) think that he will not last very much longer on his own. Things would be so different if he lived closer. Driving 6 hours one way is too much to take care of him the way he needs.
Im sorry, I know this post really has nothing to do with building a home, but Ive reached a hot button and had to vent about it. Really what this means is that no matter how many bedrooms or how much space, I HAVE to be ready in case he decides to live with me.

You certainly do have a lot to think about, and as others have noted there are many competing factors to consider, too. My husband and I finished our build two years ago (it is so worth it but the way!) but I wish I have been thinking about more "ifs" when we designed our home. We are now unexpectedly pregnant with our third child very close to the second, and I already wish we had another bedroom ready near our master bedroom. Although I may not have changed the plans, I wish I had spent the small extra dollars to run plumbing to spots that could then have been more easily renovated to carve out new spaces. So, you might consider some options between extra bonus rooms or no extra rooms, like running plumbing and planning for where walls could come down/be put up in the future, In other words, you'd still do some renovating maybe, but at least it would be relatively inexpensive. And if the "maybes" don't happen, then the cost to prepare was minimal. We did a version of this by planning for a full bath on the first floor of the house, even though right now we only use that bath as a powder room. I wish I had done more of that planning now that we will be looking at a more major renovation within 5 years. Good luck, I know it's not easy!

@petesmom- so sorry to hear about your struggles!

Petesmom- I am so sorry to see that your grandfather is not doing very well. If he doesn't want to leave his home...is there any chance you could have someone come in a take care of his house? I don't know where you live, but in Washington state we have several programs that he might qualify for, if the family is not able to take care of the situation or lives too far away to help.

He probably loves his dogs and wants things to stay as much as possible, the way they were when your grandmother was alive. That's understandable...frustrating, but understandable. Maybe the family can talk to him and explain that for the dogs' good (as well as his own) it would be best to keep the house in good condition.

If your grandmother was able to keep things going, hopefully a bi-weekly visit from a housekeeper and maybe something like 'Meals on Wheels' will help him, at least for now. If things get worse, then maybe he'd prefer living with you than to living in a nursing home. Most family members would...but only if they HAVE to leave their own homes.

Good luck with all this and I hope your grandfather realizes what a big heart you have and how good it is that you're watching out for him. Design your home with enough room for the 'ifs' and maybe it will all work out. I hope so :)