Before you start calling architects, it's a good idea to have some ideas on what you want. You should be collecting ideas from magazines, the web, and by taking pictures of additions on other people's houses that you like. You should also have a good idea of your budget which I'll post separately on later.
But, for choosing an architect, it's probably most important to have an idea of what you want to get out of using the architect. Her are some considerations for choosing the right architect:
1) Creativity. In general, an architect's creativity is a must for major home additions. This is how you avoid adding an uninteresting box onto your existing home. You should be looking for someone who can creatively solve design issues in your house without blowing your budget on a single feature.
2) Budget Mindfulness. Although your budget may not be set in stone when you first interview architects, you should choose a number pretty soon after. Your architect should know what that number is and should be aware that certain design choices may dramatically affect the budget. Although I'll dedicate a separate post to budget, your architect should be aware of your budget and help you stick to it, no matter how large or small.
3) Experience. You should also look to hire an architect who specializes in residential design and who understands the needs of your household either through personal or professional experience. Architects who focus on commercial design may not have the most relevant experience for your residential home addition. You should be comfortable with the number of years of experience they have. For example, we chose to interview architects with a minimum of 10 years of experience. Also, the architect should have significant, recent experience doing similar projects in your jurisdiction (city/county/town) because the codes vary greatly by municipality and can be costly to change plans later. Some architects also choose to hire building engineers to confirm the structural soundness of plans requiring steel, underpinning, or other serious structural issues. This is normal and the homeowner should welcome this input because it lessens the likelihood that your plans will get bounced during the permit process.
4) Billing. You should understand how the architect bills you for the project and this should be clear in any contract you sign. Some architects charge you based on percentage of building costs, and others charge per hour. We wanted an architect who charged per hour because we felt we had more control over the costs. Also, architects who work for larger firms probably charge more than those who work out of their homes. Our architect works out of her house and her billing rate reflects that.
5) Personality. This may seem realy obvious, but you should really like the person who you choose. You may love the samples of their work, but if you don't really like the person and cannot see yourself spending a lot of time with the person, then don't hire them. You will spend countless hours working with your architect both in-person, and by phone and email over a long period of time (probably 18-20 months for us when all said and done). You should also feel confident that the architect will listen to your ideas and requests and not be unnecessarily dismissive.
There are probably other things that you should think about (feel free to post!), but this is a good starting point for interviews with architects.