Use existing lots
ADUs add a home where land and infrastructure already exist.
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ADUs
Let homeowners add a home
A backyard cottage, garage apartment, or basement home can house a parent, a renter, a caregiver, or the next generation.
In plain English
Let a household add one or two small homes on a lot it already owns—inside the main house, over a garage, or in the backyard.
How it works
Remove the barrier. Let homes respond.
Lower project scale
Individual households can participate in adding supply.
Expand flexibility
A home can serve relatives, caregivers, renters, or changing family needs over time.
The implementation test
Real reform or paper promise?
An ADU is a real home, not a loophole. It still follows building, fire, sanitation, and safety codes; reform removes unrelated barriers to who may live there or park there.
TestStrongWeak or symbolic
Permission
StrongAttached and detached ADUs are allowed by right.
Weak or symbolicNeighbors or a hearing body can veto a compliant home.
Household rules
StrongThe owner may rent either home to any household.
Weak or symbolicOwner-occupancy or family-only rules restrict ordinary use.
Delivery
StrongClear plans, fees, and review timelines make small projects manageable.
Weak or symbolicBespoke review costs as much as a larger development.
Strong policy principles
- Allow attached and detached ADUs by right.
- Remove owner-occupancy and family-relationship restrictions.
- Use clear size, height, fire, and utility rules.
- Offer pre-approved plans and predictable fees.
Watch for
- Mandatory replacement parking
- Utility fees resembling a second full house
- Discretionary design review
- Rental prohibitions
Pair it with
- Pre-approved plans
- Low-cost homeowner loans
- Technical assistance
- Fee waivers for income-restricted ADUs
Measure whether it works
- Applications, permits, and completions
- Median approval time
- Project cost
- Long-term rental availability
In practice
What this reform looks like on the ground.
Seattle, WashingtonAugust 8, 2019
Ordinance 125854
Seattle allowed two ADUs on eligible lots, removed owner-occupancy and off-street-parking requirements, and relaxed size and location rules.
Lesson
An ADU reform works as a package. Allowing an extra unit means little if household restrictions, parking, or physical standards still prevent ordinary projects.
Evidence status
Seattle ordinance and legislative record ↗ (opens in a new tab)Seattle's ordinance and annual ADU reports document the rule change and the permitting response. Comparative ADU research explains why parking, occupancy, fees, and design rules work as a package.
Why support it?
The evidence behind the solution.
Strong evidence
A city cannot build homes it has made illegal.
Legalize real projects across enough land and builders respond. Auckland did this at city scale and saw a large permitting boom; narrower reforms have usually delivered less.
Keep in mind: Permission is necessary but not sufficient. Financing, land, construction costs, approval time, and demand determine whether newly legal homes are actually built.
Strong evidence
Legal on paper is not the same as buildable in practice.
A reform can promise density and still bury it under tiny maps, parking ratios, setbacks, fees, hearings, and delay. Good reform makes ordinary projects legal, feasible, and predictable.
Keep in mind: Chicago's narrow reform produced no detectable building increase in its early years. That helps show why scale, feasibility, and follow-through matter.
Established evidence
Free parking is paid for through everything else.
A parking space consumes land, concrete, and money. Mandates hide that bill inside every home—even when a resident would rather have lower rent than a place for a car.
Keep in mind: Removing a mandate does not ban parking. Builders can still provide it where residents will pay for it, while cities manage curbs and accessibility directly.
Figure 2 · Living space after upzoning
The strongest post-reform increases appear after roughly five years, especially for large upzonings.
REDRAWN FROM STUDY DATAFigure 2 · Living space after upzoning
| Years relative to upzoning | All upzonings | Large upzonings | Small upzonings |
|---|---|---|---|
| −4 | -1 | -2 | 0 |
| −3 | 0 | -1 | 0 |
| −2 | -1 | 0 | -1 |
| −1 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 |
| +1 | 0 | 1 | 0 |
| +2 | 1 | 2 | 1 |
| +3 | 2 | 4 | 1 |
| +4 | 3 | 6 | 2 |
| +5 | 5 | 9 | 3 |
| +6 | 6 | 11 | 4 |
| +7 | 7 | 13 | 5 |
| +8 | 8 | 15 | 5 |
| +9 | 9 | 16 | 6 |
Figure 6 · Estimated treatment effects on dwelling permits
The estimated effects become positive after the reform for both attached and detached homes.
REDRAWN FROM STUDY DATAFigure 6 · Estimated treatment effects on dwelling permits
| Years relative to reform | All dwellings | Attached | Detached |
|---|---|---|---|
| −4 | -0.05 | 0 | -0.02 |
| −3 | 0 | -0.02 | 0 |
| −2 | 0.02 | 0 | 0 |
| −1 | 0 | 0 | 0 |
| 0 | 0.1 | 0.05 | 0.04 |
| +1 | 0.25 | 0.15 | 0.1 |
| +2 | 0.45 | 0.3 | 0.15 |
| +3 | 0.75 | 0.55 | 0.2 |
| +4 | 1.15 | 0.85 | 0.28 |
| +5 | 1.45 | 1.05 | 0.35 |