red_lover - ours looked like that at the top and when my boys were young my hubby had to make a contraption to avoid drilling into the banister and then we drilled the swing part into the drywall on the railing side. I wish I had a picture but I don't. We weren't comfortable with a compression fit for DOWNstairs - think it would be okay for going UP. That said I think it depends on how frequent your little one will be over. My sister's kids are younger than mine (lives far so not frequent visits) and for her visits we never had a problem just keeping an eye out. They weren't really too interested in the stairs with all of the activity of the older kids going on.

How wide is the aisle where the doors are? I just tested this out with my DH. I had him go down the basement stairs and come up, opening the door. I guess if he flung the door open (why would someone do this?) he might hit someone standing there who doesn't hear him coming up the stairs (deaf?), but there is zero chance he could fall down the stairs. I actually pushed on the door, slamming it shut (I didn't tell him I would do this, he wasn't happy!) and he said he was never off balance, never had a chance of falling. Test it out, there is no risk unless the person coming up the stairs is fragile and the person already in the hallway is malicious. If someone flings the door open someone in the hallway could be hit, but this is a danger with almost any solid door. Rather than posting the entire plan, copy it into Paint and then just copy the area in question into a new Paint window, then save as a jpg file.

French doors would be fine as long as they did not open over the stairway and were not in the way when open. What you are trying to prevent is heat rising by convection to the upper floor so all that is needed are doors or heavy curtains that will prevent excessive air flow. The rate of heat transfer is proportional to the temperature difference so the heat that is transferred through the doors would be minor unless the upper floor will be below freezing or leaks air a lot.

Just checking on the cherry. Can't say I'm too fond of oak myself. There are few instances where balustrade removal and replacement work out too well unless you're disassembling the stair re-milling and reassembling it. Even then unless you are talking about a stair that's made out of a rare or exotic lumber that would exceed the cost of the time and labor of doing so It is not often the best practice. I would still use care taking it apart and use it to make something else. How solid are the newels, are they loose surface mount or do they go into the floor? Iron is nice. You can either use a metal rail or wood on top of it. For a long while iron/steel balusters were a bit less expensive than wood but now they are comparable. The wood rail will usually be less expensive than the metal and easier to install. Newel posts can be either as wood newels match the iron quite well. A few of the things I was mentioning about not being correct in the drawings were the balusters and newels. The newel is an Arcways product whose turning is sized for the 30" rail heights of days long gone by. Note the extremely tall base. The balusters are StaiParts Inc. made for the same. The upper turnings don't even come close to matching the rake of the stair which is common for too many stair part manufacturers. Even with the stair and balcony rail set to the highest height in order to use a closer matching baluster combination to make it less noticeable. Arcways however does have some fabulous rails and fittings, checks their small orders for matching, offers cherry as a stock item and will sell directly to homeowners. They also carry iron and steel balusters. I believe OakPoint sells direct and Crown sells through Brosco/Brockway Smith, 84 Lumber and even a few Lowe's locations as well as numerous stair companies. Use the dealer locator on their website. Both have a good selection, have cherry as stock on some of their profiles, good quality and a product line that looks correct for todays codes. "Solid" or one piece treads sound nice but they are not really what you want unless they have relief milling on the underside and even then it's not much of a guarantee against checking or cupping especially if they are only 3/4 thick". How will the 1 1/4"+ overhang hold up? Construction adhesive and a finish nail through the top of the finish isn't going to last too long and will squeak like crazy after a while. A typical tread is 1 1/16" thick and will be made of three to 5 pieces. Cherry treads are most often made "in house" from random width/length material and a good stair shop will cut out any sap wood, match the grain and the leading piece will be well beyond the riser. Engineered treads are also available but like everything they have their pros and cons and I think the cons still outweigh the pros. The open left end should have a return nosing mitered into the front left corner and extend past the next riser by the same distance as the overhang with a scotia below. That is the dark piece I am showing on the edge of the tread in the image above. If you go through your phone book or search for stair companies or stair manufacturers and not stair part manufacturers you should find a few. If you call a stair parts manufacturer they will direct you to someone in your area that they sell to. I was looking for the dimension of the face of riser to face of riser or the run. I'm assuming 7" is the height? Floor to floor height. From the top of the plywood where the upper stair lands to the top of the vinyl on the landing and from the top of the vinyl on the landing to the top of the vinyl(?) in the basement. If the vinyl is flat I would agree with keeping it in place if the glue has failed and it's lifting remove it and in either scenario use a quality felt paper underlayment. Did you already place the order with the millwork shop? How soon do you need to get this done? I really think it would be in your best interest to explore all avenues and plan out the whole project before you buy anything. As Casey noted above it is often cheaper to remove all and replace. It would take you less time to install the two units than it would to even do one single tread.