Firstly, without a design and detailed spec. all you'll get is a rough rule of thumb on cost, no more.
Lots of factors impact on cost - the geometry of the building, how much articulation the facade has, how many windows and their size, whether you're building on flat ground or a slope, is the building single storey, two storey or more, the quality of materials, fixtures and fittings, etc. Normally I wouldn't include a septic system, road access, etc. into a rule of thumb for cost but some builders might.
Guidance I give my clients is anything from 200 - 500 is a reasonable range but we've one contract on the books at the moment that is costing over $1,000/sq.ft.
First step in the process is to prepare a BRIEF. Regard this as a wish list and a written description of what you want. You can prepare it with your family and it will help you crystallize your thoughts on what you want the building to be. I can send you a model brief I give clients with some questions and answers but the brief is very important - it's purpose is to define the questions to which the building design is the answer.
On military projects, the brief for a building is incredibly detailed and can run to several volumes - these guys really define what they want - but on a house it need be only 4 or so pages. Once we have the brief, I ask my clients to sign it because that's what I use as the basis for preparing the design. If the client asks for changes as we progress, I update the brief to reflect those changes.
Some of the questions the brief will ask: how many rooms, how big, open plan or cellular design, do you have a preference for one or two storeys (or more), what image, appearance and quality do you have in mind, is there likely to be a need for future expansion, what type of heating system, any fireplaces and if so, what sort, any need for a/c, etc.
Once you have the brief fleshed out, that's the time to begin the design. Begin the design without a brief and you can waste a lot of time preparing drawings that reveal what you don't want. With a good brief, the design needn't take long. With the design AND a specification, it's possible to prepare a detailed cost plan.
Cost plus contracts are one way to go but I'm wary of them. I've worked on cost plus contracts and they can abused. I built five military airbases in the middle east on cost plus 14%. Where one filing clerkess would have been enough for the office, we had five. I shared a five bed villa with one other guy and we had two house boys and a cook, all cost plus. I'm not saying all contracts are like this, they're obviously not, but generally cost plus gives no incentive to the builder to shop around on your behalf for best price materials or components. The more the job costs you, the more he makes. Rather than cost plus, I prefer a fixed fee, negotiated before the contract is signed but for a known design and specification. With a fixed fee the builder has no incentive to drive the cost up but gets a fair profit for his efforts. That's how we work. The fixed fee doesn't include managing a lot of changes. It will accommodate some but if during the works I'm involve in a lot of extra work moving walls, windows, adding features and fixtures here and there, the fee will adjusted to reflect the additional work. That's only fair.
As others have advised, getting the right builder is key. Good idea by army grunt to speak to the local building inspector. They have their finger on the pulse and will normally give you a list of three or four good contractors you can approach. Ask builders for references, speak to previous clients, go visit some houses. Most people are really proud of their buildings and will gladly show you around. Ask them about problems during construction, how was cost managed, was the building delivered on time, what were the problems along the way, what were the post construction defects that arose and did the builder attend to them in good time?
A good builder can save a lot of money on small items if he shops around for best prices. We get competitive quotes for all materials and components and over the lifetime of a job, that $50 saving here and $60 there will add up to thousands of dollars. So make sure your builder has a cost management policy in place and ask him for details of how he implements it. If he looks blank, he hasn't got one.
Last point, buildings are like anything else - you get what you pay for. We can't build silk purses for the price of sow's ears. When I'm faced with clients who have unreasonable expectations and think I can, I ask them what car they came in. When they tell me its an Audi or a BMW I ask if they got it for the price of a Pontiac, because that's what they're asking me to do in building terms. If they want their building to be a Merc, that's what they have to pay for, although I'll build them a Chrysler if that's what they want. I'll get them best value for that Chrysler but pulling rabbits our of hats isn't part of the service.
