Green glue comes in 24oz tubes, like caulk. You put 2 or 3 tubes per sheet of sheetrock, press and stick.
Keep in mind when you add a layer of sheetrock, it will change the door and window trim requirements. In a spec home, that will be tough to finagle. I was on good terms with my builder and he was willing to make the trim flush with wall but was concerned at how it would look... and it was 'my house' so he'd do whatever... but he wasn't able to re-order doors that could accomodate thicker walls. If the wall has no trim, there shouldn't be any problem. You could even show up the day they're doing it with green glue and have the workers put up another layer and get away with it for next-to-nothing. (I found cash and beer to be persuasive for my 'custom requirements')
The key to your house is to help manage it without the builder knowing it. Let him be in charge and only suggest things when they're really wrong.. and don't try to beat the guy up unless he's being a complete tool. Stick to your guns about what you want for sure.. but being a friend and using common sense got me lots of leeway for doing what I wanted during the build... and after. I'm still on good terms with my builder and salesperson... and that has helped me feel great about the whole situation... even if I did pay too much.. ha.
A lot of people use scare tactics and try to be an ashole to get their way.. and with some builders that's what you have to do. If you're smart you can get what you want in a lot less stressful way... and everyone feels like they came away winning.
Simple stuff I wish I knew... Where the main water comes into your house is important. If it goes straight through the middle of your front yard, then enters your house under the sidewalk, that doesn't leave you much option for adding an irrigation system... Our check valve is in the middle of our front yard. Also, you can tell them where to put the spigots... and pay on the spot to add another one if you want 3 or 4..
I got lucky on power.. It enters the house about 10' from where I plug in my amp.
Be there when they're doing anything you care about. The electrician wires based on his convenience at the moment, not with thought as to how you'll use the house. Boxes get put too far from the door, switches are wired all sorts of ways... and they're just retarded with placement a lot of times.
Plumbing inside the house was fine with me. I hired the same plumber to do the basement bathroom. I wouldn't let the electrician back into my house to do work if he paid me.
Sound proofing is very, very difficult to get right in the real world... especially if you're dealing with a builder that has a formula to build that house. Sometimes you can change things a bit but sometimes you can't... it may not work with the plan. Best of luck adding sheetrock during a build... or anything like that.
I'd recommend running all of your speaker wires (cl3 or better rated in-wall wire) prior to move-in. Know your local code if you do anything... so you don't slow down the progress or get your wrist slapped. This is your builder's house until you pay for it. But it's really your house so you have to balance this stuff... hence the reason I suggested building a relationship with your builder.
OK.. back to work.. I could go on all day (obviously)
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Well, one more thought... pay extra for a good HVAC system. Get the Lennox Signature series. Do a 'custom request' and at least get a quote.