There was a home there before. They may have had an outhouse. Maybe there is not enough room for a conventional septic system on the lot with today's rules. All is not lost. You have alternatives.
1. You may be able to have a grey water drain installed for your wash water. The most common brand is an Elgin In-drain.
2. You can install an incinerating toilet which needs no leach field at all. I have an Incinolet at my camp.
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3. Although the local planning board is limited by state guidelines, you may be able to get a variance from the local appeals board for a 2 bedroom.
I have been on planning boards in Maine since 1973 (yes, 35 years). PM me with any questions involving companies. Right there in Etna there is a company that makes portable buildings. I own one. They do excellent work and are quite reasonable. Their biggest is a 14 by 40. Most towns don't require a building permit if the building is not on a permanent foundation. After all, it's portable.