I agree with some of the other responses. Depending on the size and type of porch and garage you're looking to build, if you only need 35K or less, I suggest you get a personal line of credit loan rather than a mortgage. If you have solid credit, income/job, and savings, you should be able to get a great rate and terms comparable to or even better than a traditional mortgage. It will also require less paper work , time, and expense and not put your home in jeopardy.
Furthermore, although the loan itself will probably not be tax deductible, if you keep the receipts of all the improvements or additions you did, you should be able to write/depreciate a large portion or all of it off. But consult with a local tax consultant for your own state specific requirements.