All players are important. Don't try and do this cheaply, it is a big project what you are talking about. Within budget yes. However taking on such a big project you may not know at this point how much it will cost, your budget could be unrealistic. I say this with much respect, not disrespectfully :) ask your architect if they can refer you to people to talk to. General contractors for example to take on the project ( some can ) then they sub contract jobs out, like plumbing, electrical etc. My advice would be to find a general contractor, actually three, so you can have three options and discussions with how they would proceed ( I always say three is just enough to go by ) you can also ask who their sub contractors are and look them up. With the Internet now reviews are all over the place. Ask friends or family if they know of someone.
I hope this helps.

Do not hesitate to ask your architect. Architects are professionally trained to see in 3D and to look at the big picture/project. There are many good articles on hiring an architect and dealing with contractors on HOUZZ. A good architect should make you feel better about your entire design process from start to finish. ASK many questions and ask the architect about overseeing your Design build process. Hire an architect who knows construction and design, not just design. Good luck. HOUZZ is a great resource.

Agree with Mark IV, at the very least have a contractor involved from the start especially on a addition. The contractor will see things that the architect will not, be able to problem solve and suggest how thing to be designed/engineered before hand. Most free lance architects do not have "build" experience, (there are exceptions to this) design-build is the way to go on additions/remodels.

Remodels and additions are much more challenging than new construction because you'll need to work around a greater number of constraints including some that may not be identified until you start doing the "surgery." The best way to ensure you achieve your budget objective is to select your remodeler and architect and have them work together from the outset. Alternatively, you might find a design/build contractor with both competencies under one roof.

It is not uncommon for remodels to cost double/sq.ft. as new build.

I vote for a design+build firm that is qualified and experienced with the "un-do, re-do and new-do" requirements of remodeling. The objectives and details of the project are understood across the board at all times. Realistic budgets are developed in tandem with design so as to avoid going too far down the design path with high expectations only to be shattered by high price tags. When there is a need for modifications in plan or process, responding to them and their financial implications are immediate. Responsibility is under one roof. Contact with the homeowner is direct. As a result, the project is shorter in duration from planning to completion. This approach is an efficient use of resources and maximizes time which translates into better value for the dollar spent and earned.

Brought to you by...Scott Design Inc...a Design+Build firm...of course :)

P.S. In my area, many of the architects do not like to get involved with remodeling due to its limited budgets relative to new construction. And general contractors do not like to get involved with developing design because it's too time consuming. They would rather do the work of construction and follow the plans and specifications to maximize their time. And, if an architect and GC become involved, they prefer that the homeowner determine many of the "finishes" . Again, to maximize their time. Typically, this is where the homeowner needs the most guidance.

Our firm can handle the details if the homeowner wants that written in the contract. We are construction oriented, having worked for builders and developers most of our career. The devil is in the details, yes we agree. A good GC or Design/Build firm is good too. It's up the homeowner's budget, needs and comfort level. And we agree, that the contractor comes across the real construction issues. Communication and customer service is 99% of every project. Good luck. Happy Houzzing.

Is there a small part of this renovation such as replacing a bathroom that can be done separately first? If you have never done a very large project, it's good to get some experience with design, budget and construction.

Allow 20-30% contingency: 30% for old houses where there are surprises. Expect code updates may be required for some areas, check with city hall before you start.

You shouldn't need to allocate as much as 20% to 30% of a large-scale remodel budget for "surprises" if you engage professionals who verify existing conditions, produce accurate plans and a well-defined scope of work. If you want to minimize "surprises" you will be well served to involve the contractor who will be performing the work from the outset.

Contingency in budgets can include more than surprises. It can also include money for change orders such as more expensive cabinets or adding a skylight. I agree that a 20-30% contingency can be reduced in the circumstances Charles Ross Homes describes so clearly.

I've seen designs raise the budgets as much as 20 or 25%. Once you've gone to construction contract, budgets shouldn't rise more than 5-7%. If they do, someone didn't do their homework upfront.