A common mistake people make when serving as their own General Contractor is to hire the first and only person they meet on any given trade. Roofing, ceramic tile, or the septic tank installation; it doesn't matter...you have to push hard to get multiple quotes and resist the urge to take a short cut and move ahead with less than three.
Let's use plumbing as an example. The rehabber and Joe Plumber meet and tour the property. They talk about the project, perhaps review some drawings or sketches together, and answer each others questions. This leads to an estimate and the novice renovator has to make a decision. First of all, they don't have to tell the plumber yes or no at this point. However, what happens many times is that the inexperienced renovator jumps into an agreement too quickly.
I know how this goes in the mind of the renovator or home-builder because I've been there hundreds of times. The plumber seemed like a good guy that knows what he's doing and what he's talking about. Maybe he came highly recommended and that adds to the impulse to give him the green light. Renovating a house is a lot of work with a long list of things to get done. The person making the decision is focused on crossing things off their massive to-do list and having a plumber on board (with a name, phone number, and agreement) would feel like a big step in the right direction. Plus, it's easy to award the first plumber the job and rationalize later that his competition would have likely been in the same ballpark (price-wise). This is totally wrong. You shouldn't stop at one quote and settle for that because one estimate won't give you an accurate understanding of the value of the work. It's tempting to move ahead with the first price from contractor #1, but it's a mistake even if your uncle (a retired plumber) told you how much the job should cost, you got a guesstimate from a helpful licensed GC you know, or you diligently researched the subject on the internet. Those are all smart steps on your path to finding a plumber and signing him on, but you need to keep going.
Let's say the first plumber (that good guy who seems to know his stuff) offers to do the job for $4,150. You keep at it and plumber number two hands in a quote for $3,300, a third company says they'll do it for 3,000, and a fourth man submits a $2,500 estimate. Three bids is good, four is better, and five is great. For the sake of this example, let's just say a fifth quote comes in at $3,250. This let's us know that the plumbing work for this job is worth a little over three grand. It's not uncommon for the bids to stack up this way, and I'll also point out that I don't always take the lowest price. I'll look hard at all the proposals, but ultimately I want a trades-person who can perform.
A construction budget can get out of hand quickly and awarding work without getting plenty of prices is oftentimes a big part of this. Going with the first quote would have meant overspending by a thousand bucks. If you do something similar on all the major trades you'll be hemorrhaging money unnecessarily and budget headaches make any project harder than it needs to be. I understand that there will be rehabs and scopes when five prices is just not doable; rural areas off the beaten path make magic numbers four and five a real challenge, some scopes are specialized and you'll be thankful to just get two bids (i.e. stained glass window repair), and sometimes it won't make sense to beat the bushes for prices when the cost is minimal. I get all this, but really try to get three numbers from qualified folks and if you're able to get four or five on the big ticket things (like demolition, framing, roofing, drywall, painting, plumbing, electrical, HVAC, etc.) then do it.
Finally, there's a second very important reason why you really must get more than one quote per trade. You have to be ready with a Plan B. What if you choose someone and they have an accident or some personal family issue? Things happen and you've got to have an alternate in mind so you can keep moving forward if something unfortunate occurs. This preparedness is just smart business. Like it or not, the sheetrocker, painter, and electrician will sure see it like this (as business), so you need to be ready to see it this way too.
Don't be foolishly impulsive and don't get caught with your pants down. Get those quotes.