It depends a lot on what you are comfortable with. We did a minor addition a few years ago - not even really an addition, but adding a room in our garage (was a 2-story garage, we just added in the floor of the new room, basically). When we did that one, my DH and a contractor friend did all of the work. We did pay a structural engineer to check the loads and had to add a cement post.

Now we are in the midst of a more major remodel, where we added a bedroom/bathroom off the back of the house, also redoing the kitchen and family room. For this remodel, we are using a design/build firm. Anyway, we have kind of seen both ends of things.

I think your local forum is right that you need to have a set of plans. Our design/build firm had a process where they first drew up plans for us, and then if we wanted we could pay $3000 and the plans were ours, and we could take them to anyone. Or if we stayed with them (which we did) then the $3000 went into the price of the remodel. I don't know your house of course, but especially for the more complicated remodel I think you do want an architect, especially since you'll be doing plumbing for the bathroom.

In our case the convenience of the design/build firm was a factor that made it worth paying a little more. To some extent as you said it matters how involved you want to be. But it's not that we didn't want to be involved, but that we wanted to concentrate on functionality and design choices but not worry about coordinating schedules for subcontractors and babysitting permits through the county offices. DH is over there every day now that we're in progerss, so we're definitely involved, just not in the management part of it.

We recently added a sunroom and breakfast nook to our home. If you want to read our saga, search for two posts whose titles begin "adventures in contracting."

In short, my advice would be:
1. Have plans drawn up by a pro. That'll ensure you hash out all the details up front (much cheaper to change on paper than after framing is up) and have structural calcs done as needed. It will also make it easier for contractors to bid on it, as they won't need to make assumptions.
2. Talk to multiple contractors to find several that you like, and ask them to bid. Take lots of time and care selecting one. Find one who focuses on projects like yours. A design/build firm that's used to large projects may not be sensitive to your budget, and a new-home builder may not be as skilled (or as patient) with tying in the new to the old.

A word about plans: our sunroom plans were very detailed and cost about $1500. They didn't include mechanicals, because we're DIY-ing them. But the detail involved was critical in resolving a couple issues where the contractor said "I didn't include that in my bid."

First, really do consider what the additional couple of hundred grand that you'll be putting into this house would buy you elsewhere in your city. Remodeling is EXPENSIVE, messy, stressfull and may price you out of your neighborhood for the end result when it comes to resale. The biggest home in the neighborhood suffers when it comes to resale time. Talk with a real estate agent first about your neighborhood and your plans. Don't plan on recouping a dime when it comes to resale either. The days of spending 150K on a master addition increasing your equity in a home by 100K are gone----if that was ever a real life scenario in your area anyway. It was a brief blip on the national radar screen and now we're back to "normal".

But, if you've looked at all of the pros and cons realistically and still think that the couple of hundred grand spent will greatly improve the quality of your lives for the brief time you'll actually be living in that house, then be sure to involve an architect. The description of the scope of work is beyond most DIYers to be able to draw up blueprints and other construction documents for.

I am going to respond on the other side. I guess a lot depends on the two of you and also the rules and regs where you live. We just added a half second story that involved ripping most ot the roof off, putting in a new bath, replacing all the plumbing and electrical for most of the house (500 sq ft). All without a design pro. But there were several issues that made it workable.

First: In out small town, a hand drawn design is acceptable. Every item did not have to be drawn in, so long as we stated we were going to follow code (i.e. outlets, etc). It was up to us to either research it ourselves, or trust our CG, but either way, we did have to follow code to pass final inspection. Also I am a very visual person (as it sounds that you are) I had a most of the design thoroughly thought through. Also remember that no matter how much you think about it and design a head of time, in a remodel, things always change, so it helped having a constantly rearranged plan in my head. I was also available, by phone or in person, during most work days for issues when they came up.

Load issues: We have an older home that is very solid and there were no questions as to where the load bearing walls were (outside and down the middle). The GC did ask an older experienced "friend" CG to come and look the house over before he started.

All in all, we got the job done having spent about half of what a major deginer/contractor would have cost us. That is equity that would have taken us years to recoup. For us it was worth it, but we did work for it.

Thanks for the replies. :)

First, about moving...we originally were going to move. But we got a very good price on our house about 5 years ago on a very nice, private lot. Our house is about 2,800 square feet and we were looking at houses that were 4,000 square feet that were more than double what we paid for our house (even with the downturn in the economy) and most of which were newer and therefore in our area on teeny tiny little lots where they just pack all the houses in.

We realized it was more economical for us to stay where we were, even with the cost of the addition. Plus, there are other houses in our neighborhood that are, of course, not selling. Cause not much is selling these days. So I don't even know if we could sell our house. And when we looked at the other houses we realized exactly how nice our house was! We've kept it up and put in a lot of work into it over the years and many of the other houses were not so nice.

We've gotten another bid now. One was with a big company and two have been with smaller companies. Actually, the two quotes from the smaller companies were very reasonable and both in line with each other. Both came in around or a little under 100 a square foot, which was what a realtor friend of ours had said they might.

I found out that at least one of the smaller companies works with a draftsman, and I think that since we really do know we want, I think that might/should/will work out! :) We're still waiting on one more estimate and then I'd like to personally go look at some of the work these companies have done.

This is just a side note...a couple of years ago we wanted built in bookshelves in our front room. We initially called up the cabinet makers and built in companies listed in the phone book and at the home shows. We got very high prices that we just weren't comfortable with. We then found a local carpenter to use white stock cabinets from Lowes on the bottom and basic lumber on the top, which he added crown molding to, pretty hardware and some nice coats of white paint. Our built ins are gorgeous and they cost a quarter of what we'd quoted from the other companies. So I guess I'm skeptical and jaded. We're used to DIYing as much as we can, but we're not builders. I just notice a lot of my friends paying too much for home improvement projects because they don't do enough research or are too scared to involve themselves in the projects.