That bonus space is an awkward space. I would use the entire space for the master bath, and add a skylight in the bedroom for light. (Or a new window if it is an exterior wall). The trick will be to find someone who can do it all for your budget, or do it in stages.

Otherwise palimpsest's idea is good.

I love palimpsest's idea. You wouldn't have to go through the slab at all with that plan. You might still feel a little cramped on the toilet (excuse the pun) but you would have plenty of shower space and space to dry off. You could remove the existing door and narrow the doorway into the bathroom/addition a little bit and put double doors on it, and keep them open for light in the bedroom when you needed it.

I don't know if you can do it within your budget, but I would make the entire addition the master bathroom. I'd keep light into the bedroom by using french doors or other style doors with obscured glass. You might also look into solar tubes. My mother-in-law used one in a bathroom with no window and I was amazed at the amount of light it provided.

Thanks for the thoughts!

We can't really bump out the shower towards the window - because that is the wall with the water pipes. Unless we make it a bench within the shower and have the pipes routed underneath; that would give elbow room but the entrance to the shower would still be right in front of the toliet.

Our friend doesn't seem to think it is beyond our reach to move the plumbing. Hopefully he'll come over if we decide to do that.

I'm leaning towards taking over the entire addition.
Or moving the bathroom to the other side of the addition - and putting a row of windows in where the bathroom currently is (there is a lake view and we'd get morning light then).

A skylight in the bedroom would be fabulous.
I definitely would like to make the bedroom window larger, but not now. Plus we need to work on our landscaping to improve the view. The neighbor's kitchen/bathroom/bedroom look right in.

I've seen pictures of those light tubes...they seem fabulous. I'm thinking of installing on in our upstairs bedroom whose ceiling is currently out. I love energy efficiency but I also really love skylights and being able to look up at the sky (and open them).

If we take over the entire addition...do you think it is worth it to add a tub? Or would it be too crowded.
My husband decided he would want one, despite never taking a bath since I met him 15 years ago...

I sort of like palimpsest's idea, and I think it might be the best option. However, I'm not a big fan of sinks outside of bathrooms. What is the distance between the left bathroom wall (toilet wall) and the window? I'm wondering if you could make the shower a little bigger and make it a neo-angle shower so you exit into the center of the room rather than directly onto the toilet. Then put a small vanity to the right of the shower (maybe 24-36" at most). Alternatively, move the door from opening inside the addition to open in bedroom (maybe a pocket door would be possible) and put the vanity directly across from the toilet and run the shower all the way along the back wall. Again, I'm just guessing at dimensions and am not certain if that would give you sufficient clearances.

Tubs are pretty, but if you have one elsewhere in the house I believe you will be much happier in the long run with a big shower. I love my new big shower! It's so nice to have room to dry off.

We installed a Solatube in our shower and we had never done that sort of thing before. It was very easy and it does not leak (my big fear). It looks a bit like a decapitated R2D2 on the roof from outside, so keep that in mind. It really brightens the room, even though we have a wall of windows in there already. You cannot see the sky through them, though.

The lake view would be a must for me, whether it is in the bathroom or the bedroom. The bigger the windows the better!
Obscured glass doors is a brilliant idea.

Cutting through the slab and installing new pipes is not rocket science, it's just tedious hard work. If you hire a plumber to do any of the actual pipe work, be sure to check often that all pipes are INSIDE the walls and do not jut into the area where you will be attaching the drywall, and that the drains and toilet flanges are LEVEL. The screws for the toilet should be positioned correctly too. There are a lot of little measurements that can make your life miserable if not done correctly when the plumbing is roughed in. Good luck!
Renee

Take out the wall dividing shower and sink. Take out the door, then the space is opened up. I would install pocket doors or barn doors with some sort of glass- perhaps frosted, separating the bathroom from the bedroom. This way you could have it open and even when it is closed it would still let light into your bedroom.

I would expand the shower, go to where the sink was, maybe a little further. You won't have to change any plumbing for the shower that way.

I would move the sink to the east (far right) wall, maybe a double sink. I would perhaps build a false wall, run the plumbing for the new sink from the basement and then through the wall and into the false wall area.

Most plumbers give free estimates. Have a couple come out, give you their ideas and let them work up an estimate. Ask for an estimate of rough plumbing and finished so that you can decide what you want to do. If you diy, you'll save alot of money.

If you have five feet with the wall moved out to the window you could put in a shallow tub that has room for plumbing to run underneath. This is pretty standard in apartment buildings with slab construction so that your plumbing pipe isn't in someone else's apartment. That way you could move things a bit without cutting into the slab, possibly.

I'd lay it out as shown, using a combo shower/tub with glass block or something that allows privacy but also has light on one side of the shower. You'd need to move the plumbing on the shower to the other wall or maybe (don't know enough about plumbing or construction, others can help) you could have drywall in the middle and glass block or privacy type glass on the two sides. You would have to run plumbing for the double vanity as well.

I didn't think I would, but I rather like gmp3's suggestion. I would either do a regular drywall wall at the foot of the tub, or put the plumbing against the outter wall (same wall as the toilet) and do full privacy glass.

Can you add any more windows (maybe small ones even) in the bedroom? Or maybe there is one above the bed already? If you wanted to make the entire addition the bathroom, you could go with glass doors (maybe french doors - either privacy glass or regular) that you could keep open most of the time to allow in more light.