ADU Reality Check: Zoning, Utilities, and Budget Before the Floor Plan
Backyard cottages are having a moment, and so are half-finished ones. Two weekends of homework on zoning, utilities, and honest numbers decides whether yours gets built or gets shelved.
An accessory dwelling unit — a backyard cottage, garage apartment, or basement suite with its own kitchen and entrance — can house a parent, rent for real money, or give a grown kid a soft landing. It is also a full house in miniature, with every trade and permit a house needs. The projects that stall are almost never stopped by construction. They stop at zoning, at a sewer line that cannot take another bathroom, or at a budget built on a floor plan instead of a site.
Start with the rules, not the rendering
Every jurisdiction that allows ADUs writes its own limits: maximum size (often 800 to 1,200 square feet), height caps, side and rear setbacks (commonly 4 to 5 feet), lot coverage, owner-occupancy, and parking. Some cities pre-approve stock plans and waive fees. Read the ADU section of your code or call the planning counter; a half-hour conversation now saves months later.
Walk the lot with the setbacks in hand
Pull your plat and find the property pins. Stake the setbacks onto the ground with string, then subtract easements — a utility easement along a rear line can eat 10 feet you assumed you had. What remains is your buildable envelope. If it is smaller than the cottage in your head, you learned the project’s most valuable fact for the cost of twine.
Inventory the utilities honestly
Find the sewer cleanout and figure out where the lateral runs and how deep; a detached ADU 60 feet away may need a grinder pump or a new street tap. Open the electrical panel: a 100-amp service already feeding an electric range and dryer will not carry a second dwelling, and an upgrade is $3,000 to $8,000. Ask the water utility about a second meter. Skip gas — all-electric with a heat pump is simpler to permit.
Price it per square foot, then add the invisible costs
Detached new-construction ADUs run roughly $250 to $500 per square foot, so a 600-square-foot cottage is $150,000 to $300,000 before anything soft. Garage conversions run less because the shell exists. Add 15 to 25 percent for design, engineering, permits, utility connection and impact fees, and a survey. Impact fees alone can be $5,000 to $30,000, due before a shovel moves.
Decide what you can own
Site clearing, shed demolition, trenching, painting, tile, trim, and landscaping are realistic homeowner scopes that shave real dollars. Foundation, framing, rough electrical, plumbing, and mechanical are licensed, inspected work nearly everywhere. Write two columns — hire and own — and put a number on each.
Run the money the way a lender would
If the ADU will be rented, compare the all-in cost against local one-bedroom rents and calculate payback in years. If it is for family, price the alternative — assisted living or a bigger house. Talk to a lender about a renovation loan before you pay a designer; financing shapes the size you can afford more than the floor plan does.
A finished ADU without a certificate of occupancy is not a dwelling — it is an insurance problem, a resale problem, and a code violation. Do not let a tight budget talk you into a “shed with plumbing” you will call a rental later. Permit the whole thing or do not build it.
The floor plan is the last decision, not the first. The lot, the pipes, and the panel already made most of the choices.
What a green light looks like
A buildable envelope that fits, a sewer and electrical path with known costs, a budget with soft costs and a 10 percent contingency, and a lender’s number. Now hire a designer who has permitted ADUs in your city, and expect nine to eighteen months from sketch to keys. Done in this order, the cottage that gets built looks like the one you imagined.
Keep the momentum going
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