When I said I don't have the time or inclination I did not mean the info isn't useful at all, and I was not being snarky. Its just that I am not dumb enough to try to learn something on the fly, gambling with my own place, when that thing probably takes years of experience to really get good at.
Realistically, it takes about 3 to 6 months to learn the basics of acoustics to the point where it is possible to design a studio, then another 3 to 6 months to learn how to actually design a studio (in terms of structures, materials, HVAC theory, etc), then about 3 to 6 months to actually do the design in something like SketchUp. If you don't have the time and inclination to do all of that yourself, then indeed the only other way to get a workable design is hire a designer with a proven track record to do it for you. The actual cost for the designer could vary over a broad range, depending on what part of the design you need him to do.
prestigious architect draw up plans for a 5000 sq. ft. luxury home we had built, and that cost about $1,500.00 a few years back,
Sorry, but I find that hard to believe. Prestigious architects who design luxury homes do not charge 30 cents per square foot. More like ten times that. At the very low end, you might find a "not very prestigious architect" for maybe US$ 1.50 per square foot, but a big name guy doing a luxury home would be more like US$ 5.00 per square foot. And that's just an architect, not a specialized guy, such as a acoustic designer.
That said, a studio design probably won't cost you as much as Greg suggested, except in the most extreme cases for very high end, very large, commercial studios. Home studio designs are usually a lot less than that.
so I doubt I'll get hit with a quote for tens of thousands of dollars to draw up a plan for 4 inner walls and an inner ceiling [maybe].
You are right: that would not be a studio design, just a very basic isolation design, so it would not cost tens of thousands of dollars. Greg probably assumed you were talking about actual studio design, which is very, very different from just designing a simple isolation system. Of course, the isolation design has an effect on the internal studio acoustics, so it is better to hire the same guy to do BOTH the isolation design AND ALSO the studio design. It just makes sense to do that. They do go together, since decisions made in the isolation system can also affect the final acoustic response of the finished studio. You really should consider hiring the same guy to do both.
This is not exactly an elaborate setup, although I understand your point that it should be done properly to work.
Actually, it is! You might not see it that way, but from the point of view of those of us who have some experience in actually designing studios, what you are talking about is far more complex than you think. Designing a control room that meets ITU BS.1116-3 (the spec for all critical listening rooms), or EBU Tech-3276 (another similar spec), or the Genelc spec, or the Dolby spec, or any other spec that is commonly used as the basis for designing a control room, is not an easy task. Here's one such room:
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 . And here's another, that is currently in the final stages of tuning:
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21368 . But then to also make that room have variable acoustics, such that it can be used as a live room for tracking as well, is an even taller order. So yes, it is an elaborate setup, if you want it done right, to accepted standards. Of course, if you just want mediocre results, and don't care too much about quality, then this is a moot point. In that case, you don't need a design at all! Just throw up some mattresses and carpets on the walls, stick some egg crates on the ceiling, and you'll be fine!

But if you do want your place to be the best it can be, see my signature, below...
Well, I disagree with all of that, and I disagree based on decades of experience. I have done a bunch of good work in one-room setups. Good work that sounds good, was well received, and so forth. In my opinion what you appear to be saying flies in the face of experience, and even common knowledge.
Well, I in turn will have to disagree with that: If what you say were true, then high-end studios would not bother to spend large sums of money to design, build and treat their studios to perfection, and big-name producers, artists and bands would not pay top-dollar to use those facilities. If what you say were true, then Abbey Road, Blackbird, Metropolis, Galaxy, Power Station, Capitol, Electric Lady, Conway, and all the others would be out of business, since nobody would go there. Yet they exist, and are fully booked (try booking a session in Abbey Road any time soon...) If what you say were true, people like John Sayers and myself would be out of a job, since nobody would need to get their studios designed. Yet I'm pretty much booked solid the year round, and I'm pretty sure John is too.
For a "real world", and high-end, example, take a look at Peter Gabriels studio.
You might want to check your facts better! From an article on that studio: "The Big Room is designed to be one large collaborative recording space, without dividing walls and also houses two isolation booths. Adjoining the Big Room and within the old mill building is the Wood Room. This room features a more lively acoustic and a booth, mezzanine floor and movable acoustic screens." So it's a five-room studio, not a single room studio, with a control room that contains two booths, plus a live room that contains yet another boot, and the Big Room was specifically "DESIGNED" to work as both a control room and a tracking room, and even then the live room has variable acoustics...
there was a really cool one-room studio in Venice with an amazing 80 series Neve, and just an enormous amount of outboard,
From the same argument as you seem to be making, I could very well say that there is never any need for such a console, or the outboard gear, since ""people have done a bunch of good work on small compact consoles. Good work that sounds good, was well received, and so forth". So nobody needs a Neve or outboard boxes to produce great work...

And yet, the same big-name studios that spend big money on getting their acoustics right, also tend to spend more big money on high-end consoles, speakers, mics, speakers and such. Why use a U47, when you can get the "same" result with an SM-58? Why spends thousands on speakers, when you can master on headphones "real good"?
I guess it's a matter of perspective: some people want the best quality they can afford. For other people, quality just isn't that important, and "that'll do" is just fine.
It doesn't really bear on the original question though, which just had to do with how much floor space is going to be lost to the new walls.
Let me illustrate this a different way: you are an experienced producer, so what would you do if somebody came along and asked: "I wrote a song: how many musicians do you think I need to produce it?". And that's all he said. Every time you asked him more about the song, such as the genre, style, arrangement, target audience, etc., the only thing he would tell you is that it's a great song, and he wants to sing it, and he has a lot of experience singing songs, and that all he really wants to know from you, is how may musicians he needs. You explain to him that there's no simple answer to his question, since his song might need a full symphony orchestra with a full choir, or it might just need one guy on a kazoo, but instead of telling you what you NEED to know in order to give him a valid answer, he just keeps on telling you that he disagrees with you, as he's sung many good songs before, and that you really don't need to know how the song goes, or what the rhythm is, or even if it is only vocals, only instrumental, or both! Every time you ask him for more info, so you can try to help him and figure out what musicians he really will need, he instead tells you that Peter Gabriel once recorded a song without saying anything to his producer about the genre or what the song was about, and it was well accepted. And he also tells you that you are wrong to want to know about the song, because there are many other songwriters that have recorded stuff very nicely without telling anybody about their song.... You keep on trying, but you just can't seem to get through to the guy about WHY you need to know more about his song in order to answer his question. So in the end, you decide that the only thing you can actually do is tell him: "Look, the best I can say, considering that you won't tell me enough about our song to give you REAL answer, is that you might need 1 musician, and you might need a hundred. Some typical average contemporary bands have 4 members, some have 20. That's as close as I can get, and I'm just guessing, because you won't tell me enough about your song!"
So, in the same spirit, I'll answer your question: How thick does an isolation wall have to be, in total? "Look, the best I can say, is that it might be as little as 6 inches, and it might be as much as four feet. Some typical average home studio walls are 8 inches thick, in total, while others are 15 inches thick. That's about as close as anyone can guess, under the circumstances, because you won't tell me enough about your studio!".
I think there's not much more that I can say! Until you actually have a real-world scenario to show us, we can't be of more help, I think.
For a home half that size, they usually charge $5000 where I live.
At least! That would be a typical mid-range house by a mid-range architect. A big name architect doing a luxury home would likely be twice that. Plus expenses.
- Stuart -