If a single story house has been built with a 20 foot front yard setback and the code minimum front yard setback is 25 feet does that mean that the 5 foot difference would be classified as the nonconforming portion of the house ? If a second story is added to this house can it be built up to the 20 foot setback ? Or would the addition have to begin at the 25 foot setback ?
I would recommend that you talk with your local or county planning officials or zoning administrator regarding this question. I have included two links to articles located on the Michigan State University Extension Website regarding non-conforming uses, land, and structures.
In summary, based on these two articles and in relation to your question:
Nonconformities are allowed to continue into the future in the same manner and same extent as they existed at the time they became nonconforming. A nonconforming property can be sold and the new owner is permitted to continue its use in the same fashion as the previous owner without any new zoning approvals. However, if a change in a nonconforming use, building, or parcel is proposed, it must conform to ordinance requirements. The ordinance should have established standards and procedures for treatment of nonconformities, with the basic objective being gradual elimination of nonconformities.
Please consult with your planning officials or zoning administrator before having architectural or building plans prepared. This will save you money and time.
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There should be a zoning administrator, planner, or planning commission staff person who can meet with you to discuss your assumptions regarding the code.
You should not have to learn the code on your own, because there is professional staff whose job it is to review concept plans or site plans, interpreted the codes applicable to the concept plan or site plan, and provide preliminary recommendations. The City of Dearborn will make the final decision regarding the non-conformity, so it’s better to meet with them right away, explain your project and get the city’s preliminary recommendations regarding the direction you should take with this project. It will save you a lot of time and money.
Please feel free to follow up with me and let me know the results of your meeting with the City of Dearborn.