Advice on lending options?

I was going to suggest looking into the 203k loan program which is for rehab projects. Here is a link which describes the program and what projects it can be used for:

http://www.hud.gov/offices/hsg/sfh/203k/203kabou.cfm

One of the projects it says it can be used for is "To refinance existing liens secured against the subject property and rehabilitate such a dwelling".

Good luck!

You can find a wealth of information at BankRate.com. It collects data on all sorts of mortgage and savings options. Here is a link: http://www.bankrate.com/
You may want to consider either refinancing your existing mortgage with "cash out" to cover the cost of the addition or using a HELOC - Home Equity Line of Credit. In either case you are looking for the lowest overall cost to borrow including all fees. You want the ability to pay off the loan early and/or to accelerate payments without fees. This is especially true if you use the HELOC and later want to "wrap: it into a refinance to consolidate the debt.
Since rates are low and unlikely to move much lower, you may want to lock the rate with a fixed rate.
Finally, be careful not to extend your existing mortgage out too far in the future as this adds to the overall cost of home ownership with increased borrowing expense - even with a lower rate.
Do your homework before you settle on a lender.