Are you putting drywall up in these areas? There are rules about how long a supply line (for water) can be and not be connected to anything (ie, capped off). I think it is only 12 inches. If you don't have a basement plan and plan to put plumbing in it, your drain lines are going to have to be put in using a jack hammer at some later time. For the attic, without a plan, the best you can do it plan to put your future bathroom near the current vent stack/drains, or make sure there is a 2x6 in wall on the first floor near where you plan to put your attic bath so the plumbers can come in at a later time and put a drain line. You'll also want to make sure your system (main line to sewer or septic) is sized for whatever your future needs are. I am just (currently) going through a remodel where we converted attic space and added a bath. Turns out, that wasn't that difficult in my house. But, the question we got lucky on, and wasn't in my radar at all was "how many toilets will you end up with". If 3 or fewer, in my area, you can have only a 3" main line to sewer. If more than 3, you need 4". I'm not sure what is standard building these days (maybe it is 4") but you'll want to make sure the capacity of whatever you put in will be great enough for what you plan. Electric is relatively easy to put in/remodel around. Have them put some conduit into the walls from the main area to the attic and basement so it is easier later to run the wires to the places they'll need to go. Realistically, though, your basement plumbing plan is best figured out now, imo.

Here's the novice answer, so I'll probably be corrected: It's more to do with the drain layout, proper slope, vents, etc. so everything should be visible. They also look to verify the joints are all glued (purple primer stain). And sometimes a pressure test is involved to show the drain line doesn't have a leak. The fixtures don't have to be in - probably shouldn't be, in case some changes need to be made.

You can bet that retrofitting after the home is built and cutting up the basement floor concrete to do it will cost more than 5K. And the builder knows it. Builders upgrades are always more than market rate. That’s perfectly normal. It’s how the world works in everything to make their money. Sell the entry model at base price and the sunroof at 2K. But all of those upgrades are generally priced less than the demolition, hassle, and additional market price rise upgrade. And they count on that to sell the upgrades. Which is why you need that 20% contingency fee, and to start the house hunt well below where you want to be pricewise.

Our plumbing cost was $420 to change from two sinks to one. An additional sink, faucet, and sink cutout by the fabricator would have been at least that if we had replaced our original two-sink vanity with the same setup. The offset single sink option is cheaper as you'd only have to cap off the second drain and water. You could do a double light over the sink and the counter so there'd be no need to change your electrical boxes. I considered the offset option and have several photos in my Vanity and Mirrors Ideabook.