I live in SE PA too! Just West of Philly. Anyways, ...
You can almost do this on your own. Here are some of the 'thoughts' you need to reason out. First, with the addition you have in mind, are you pricing yourself out of the neighborhood? In other words, if what you're looking to build is WAAAAAY 'out of character' with the rest of the neighborhood, then you'll probably only help the Appraised Value of your neighbors' homes more than your own. At the same time, IF this addition would be considered a 'normal' transition (where, say, the other homes in the neighborhood have the lot land to do the same type of thing), then you're probably OK.
Let me give you a real example. There's a 30 year old developement of 17 Colonial-style Homes. They're ALL build, basically, exactly alike. 1 has a 2-car garage and a pool. Another home has a 450 sq. ft. addition on 2/3rds acres. The majority of the rest of the homes have finished basements, no pool, no garage, and are on less than .4 acres. Now, if you look closely at this, only 2 of the homes have some 'extras'. The remaining 15 are pretty much as they were when they were built. The building of the addition on the 1 home certainly raised the benchmark for that house. At the same time, the rest of the neighborhood figured they could probably get a few extra $1000s because 'that guy' is asking so much. So you see, if you were to do as you describe in this particular neighborhood, you may not ever recover the costs if and when you decide to sell. At the same time, you've certainly helped the property values of the other neighbors.
So where does that leave you? The bank will do an Appraisal of your home to see what it's worth, and based on how much you owe and the cost of the addition, they want to simply know if they'll be able to recover their loan should you go belly-up the day after the addition is built. A Realtor, generally speaking, is probably gonna be REAL happy no matter what you do to 'improve' you home. (It translates into a bigger 6%, that's all.) And the Municipality you live in will see more $$$ come the day they reassess your property. This leaves YOU to figure out if doing the addition vs. moving to a larger home is worth the $$$. If I was you, but I'm not, but if I was, as long as I plan on being in the home for a lot of years and I'm NOT really pricing myself out of the neighborhood, then I'd do the addition. On the other hand, if there's a good chance a job transfer may come through or you may have MORE kids than you plan, or the M-I-Law may move in, or the water in the well will dry up, and so on, then I'd consider moving. Yes, moving is quite disruptive to say the least! There's LOTS to consider - Just consider it all carefully.
I don't know of anyone or any service, in particular, that can do as you're asking (even for FREE.) If it makes any matter, folks do do things as you're describing no matter what's going on (or not going on) in the neighborhood. And that's OK w/me because if it's what you want, than you do it. That is certainly a fallback for you. If you need more help, e-mail me directly or post up.