As for the value addition? It depends on the price per square foot and bedroom count affect in your area. In general, homes with garage sell for more. Homes with 4 bedrooms sell for more and faster than 3 bedrooms. And there is value in the "time on market." So if you home sells for more in a shorter time, then you are saving mortgage payments as well.
The septic may be able to be recertified as being acceptable for a 4 bedroom... some counties rate this according to the number of bathrooms instead of bedrooms. But ultimately it based on the "maximim occupancy" for the home-- which is based on bedroom counts. Because if you can technically fit 4 people per bedroom --- and you have one bathroom-- it doesnt mean that they wont all still be using that one bathroom.
There are other ways to "go green" by using "alternative" septic systems, which allow for higher capacity in the same square footage on your lot. And there are even toilet systems which "burn" the waste immediatly. And there are also "grey water" recycling systems which will reduce the amont of shower and sink water going into your septic system. Thereby increasing the capacity rating. So, consider those options as well.
If this answered your question, please give me a "THUMBS UP" or "BEST ANSWER"-- thanks!
Lenn is correct. If you want accurate advice go to the source. The county health department is the ruling body when it comes to septic permits. In Guilford County that is the Guilford County Department of Environmental Health at http://www.guilfordhealth.org/ then click on Environmental Health Department then the wastewater management.
Or the state department at http://www.deh.enr.state.nc.us/osww_new//
Hope this helps,
If you are torn about your options to sell or stay, to make the additions or not, or just want a little more advice, I am happy to schedule a time to discussyour options further. You can contact me through Trulia and I will follow up with an e-mail or phone call, whichever you are most comfortable with.
Lenn has hit on one of the main facts you need to remember. While a contractor may quote you a price "as long as they do not have to pull permits". That is a red flag. Besides if you have all that work done and no permits are pulled you will not be able to count that square footage as finished if a broker lists your property in the future. Also one question is are you on public water and sewer or a well and septic? If you are on a well and septic what is your existing septic tank rated for? If you presently have a 3 bedroom 2 bathroom home and a septic tank rated for only a 3 bedroom, 2 bath home then when you go to sell that is the only way you will be able to list it.
Now on the value if everything is done to code and permitted then a simply broker's price opinion can be done to determine the value per square foot of your present home. That can be extended out to show you what it would be with the addition you are thinking of. Now the only problem with any answer you get, even if you paid for an appraisal, these are all just "opinions of value" and are only good for a very brief amount of time. There are plenty of people that did the same thing back in 2006 before the market fell and are wondering why they cannot simply list the added value sell their home. Any broker could give you an estimated market value for your home and then equate out what it would be with a "proposed" addition of the specified space.
Hope this helps,