How to Choose the Right ADU Size

The most common sizing mistake: asking "what's the biggest I can build?" instead of "what size serves my goal?" Bigger costs more, takes more yard, and doesn't always earn it back. Size from your goal down, not from the maximum up.

Start with the goal (Module 1):

• Rental income → 450–800 SF. One-bedrooms and compact two-bedrooms earn the best rent per construction dollar. Past ~800 SF, each added foot earns less.
• Parents → 500–800 SF single-story. Enough for comfortable one-level living with accessible clearances; more space means more to maintain.
• Adult child / small family → 800–1,000 SF, two bedrooms.
• Office / flex → 300–500 SF. Keep it small, keep it flexible.
• Maximum property value → usually the largest efficient unit, often 800–1,200 SF with 2 bedrooms — appraisers and buyers count bedrooms more than raw square feet.

Then apply the three filters:

1. The lot filter. Your buildable area (Module 2) caps what physically fits — including required setbacks and the yard you want to keep. A unit that consumes every inch of outdoor space hurts both homes' value.
2. The budget filter. At planning-level pricing (~$350–375+/SF for new construction), the difference between 600 SF and 1,000 SF is roughly $140,000+. Let that number argue with your ambitions early.
3. The daily-life filter. Picture the finished yard from your kitchen window. Picture parking, trash bins, privacy. The right ADU improves the property for everyone on it.

A useful rule of thumb: when torn between two sizes, choose the smaller one and spend the difference on layout quality and finishes. A well-designed 650 SF unit beats a clumsy 850 SF unit on rent, livability, and resale — every time.