Well, where I live an accessory structure would be subject to permits and inspection, or it would be illegal. If your lot is very small, there could be setback and lot coverage issues.
Also, your neighbors might have something to say about a 400ft2 structure that is plopped down in a day. I've had neighbors call code enforcement on me before . . . unfortunately for them, I did have the right to clear five acres of land with no permits, under an agricultural exemption
I assume this structure is going to have some sort of foundation, otherwise sound will leak like a sieve through the floor, unless you build a floating floor inside, which starts to chip away at the advantages of the prefab structure. In fact in framing a complete new set of interior walls, ceiling, and floor, it is questionable why that is a more efficient construction than framing the exterior walls to be soundproof in the first place.
Again, as an architect I'm sure you know all this, but plenty of less knowledgeable people will read this board and perhaps be misled into thinking a prefab structure is a magic bullet to avoid building codes.
And the taxes on my house are only a little more than $1K/year
You say you don't want to pay taxes on it; is that a legal exception, or avoidance? Will it be insurable without an occupancy permit?
Well, I absolutely don't want to mislead anybody, and it should be noted that not only will each state adopt it's own set of building codes, but each county, township & district... So, I'm not advising anybody to take this approach. You have to check your local codes and obtain the required permits. (Unless you work at night
)
I should also clarify that this building will require a permit, but not building permits which are a whole different ball game. This structure is deemed as "temporary" and fall under different guidelines. You are correct also on set backs. Here there is a 3' from property line set back, which in my case causes no significant problems.
The same weird thing happens here w/ deck. If the deck is "Mechanically Fixed" to your house, it is deemed a permanent structure and you're taxed on it. If you float it, you're not. (Look for the loopholes where ever you can)
I wish my taxes were 1k.. here, for my little (by most standards) lot is over 4k/yr., and climbing... These buildings are a "legal exception" because the thought is you will take it with you (though most never do).
Occupancy: Well, this I admit is a Grey Area, and I'm going to take a don't ask, don't tell stance on it. Plenty of people turn these building into spaces where they spend time, whether it be a wood-shop, or motorcycle-shop, or sewing room, Whatever... For me, I would define "Occupancy" as somebody sleeping and living in the space (I love playing my drums, but I wouldn't go that far. Although it may serve as an oasis for when I'm in the dog house for buying "Another Damn Drum Set" :Wife: )
This building can be covered under home owners insurance, but I doubt I could claim all the extra construction materials. I can however claim all the gear, and do have all my gear currently listed on my home owners insurance.
Construction: Well, lets keep in mind, this thread is going to be a step-by-step thing... Right now I'm in the "Schematic Design Phase" where we think "Outside the box" (No pun intended.. OK, well maybe a little pun). This is the phase where you start to identify problematic areas and research the solution.. it is quite possible for a problem to arise that makes your initial concept moot, and brings you to a totally different solution & approach.
As it stand now, based on my research, I will indeed be building a room inside a room, or floating it. Special attention will have to be given to the floor for both thermal & acoustic insulation, but no foundation is Req'd. The actual "Roll Off" Structure will serve little more than protection from the elements. It will have very Little acoustical value. However, regardless of building type, this is going to be the case (for the most part).
The only real building material that I've come across that not only serves as the facade of building but also has some significant acoustical value is CMU (Hollow Concrete Masonry Units). And the reason for that is the air gap in the material and sheer mass... In most cases, you really need to separate (or Isolate) your Studio Space from the Main Building anyway (and it's structure).
Additionally, I never said this method is a magic bullet or going to be more efficient, or would be right for you (proverbial)... as a matter of fact, if I did either of the 2 other methods I mentioned DIY, It would probably cost much less (Initial Cost)... I did however mention that one of the primary objective for myself was it to be "portable"... the other 2 methods do not allow this. And, I don't need a "Huge" space (It may be nice, but I don't need it). The room we practice in now is only about 11'x14' and serves our purpose just fine... This will be a large step up. Identifying realistic goals and needs is an important first step.
But again, I'm in a schematic design phase and I'm "Thinking Out Load" in this thread In hopes that others may read it and will provide real time input. You all are more experienced in the subject matter of sound & how to treat it, and "What you did" to find a solution to this problem or that is invaluable to me...
Gzzzz.... all these comments, I might think you were a building inspector in real life. (Kidding)
