Town Permit Guide

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Can I build an ADU in Hillsborough, California?

Hillsborough, San Mateo County · Ordinances checked 2026-10-05 · Reviewed · 16 cited sections

Allowed with a permit

Yes. ADUs are permitted on Hillsborough residential lots with an ADU permit and building permit, approved without a hearing within 60 days. A new detached ADU up to 800 sq ft and 16 ft tall with 4-foot side/rear setbacks qualifies under the state-standard track; larger units (up to 850 sq ft, or 1,000 sq ft with 2+ bedrooms) must meet extra local design standards.

PermitsADU permit + building permit (ministerial, no hearing) [2]
Decision deadline60 days after complete application [3]
Detached ADU (state-standard track)Up to 800 sq ft, 4-ft side/rear setbacks [6]
Detached ADU height16 ft (higher in limited cases) [7]
Max size, class 2 studio/1-bed850 sq ft [8]
Max size, class 2 2+ bedrooms1,000 sq ft [9]
Owner occupancy (ADU)Not required [10]
Minimum rental term30 days [11]
Parking (class 2)1 space per ADU or bedroom, whichever is less (many exemptions) [12]
JADU max size500 sq ft, inside the house [15]
Impact feesNone under 750 sq ft [16]

Where ADUs are allowed

Accessory dwelling units are a permitted use in all three residential districts (RD-1, RD-2, RD-3) [1]. Every ADU or JADU needs an ADU permit plus a building permit, reviewed ministerially without a hearing [2], and the town must approve or deny within 60 days of a complete application [3].

Class 1 (state-standard) ADUs

On a single-family lot you can do one converted ADU (inside the house or an existing accessory structure, with up to 150 sq ft of expansion for access) plus one JADU, and one new detached ADU [4]. The detached unit needs at least 4-foot side and rear setbacks [5], must be 800 sq ft of livable space or less [6], and is generally limited to 16 feet tall (18-22 ft in some transit or deep-setback cases) [7]. A JADU is up to 500 sq ft inside the house [15].

Class 2 (locally regulated) ADUs

Larger or attached ADUs go through the class 2 standards: max 850 sq ft for a studio/one-bedroom [8] or 1,000 sq ft for two or more bedrooms [9]; attached units are also capped at 50% of the existing house. Class 2 units need 4-foot side/rear setbacks and a 20-foot front setback [13], must keep total lot coverage at or under 50% [14], and must match the main house's materials and colors, use down-lights, provide evergreen screening, avoid direct sightlines into neighbors' windows, share the main driveway and have no roof deck. One parking space is required per ADU or bedroom, whichever is less [12], with exemptions for units within half a mile of transit, conversions and others; parking lost to a garage conversion need not be replaced. Projects that don't meet the objective standards can seek an ADU exception through design review (a noticed "type B" project).

Occupancy and rental

There is no owner-occupancy requirement for ADUs [10]; a JADU that shares a bathroom with the house requires an owner to live on site and a recorded deed restriction. ADUs and JADUs can't be rented for under 30 days [11], and owners must report actual rent annually. No impact fee applies to a JADU or an ADU under 750 sq ft [16].

State law

Chapter 17.52 was re-adopted by Ord. 800 in April 2026. The class 2 front-setback and lot-coverage limits apply "notwithstanding" the 800 sq ft guarantee [13][14]; state law bars rules that block an 800 sq ft ADU, so state law may override those provisions on a tight lot. Starting January 1, 2027, state law (AB 956) will require cities to allow two new detached ADUs by right on a single-family lot; the town code currently allows one.

See how Hillsborough compares with 8+ other California towns on accessory dwelling units (adus) →

Next steps

  1. Decide whether your project fits the state-standard (class 1) track: conversion, or a detached unit of 800 sq ft or less, 16 ft tall, with 4-ft side/rear setbacks.
  2. Prepare scaled plans (site plan with fire access, floor plans, all elevations, two sections, materials/colors) plus title report and survey information.
  3. Email the Planning Division's ADU specialist (adu@hillsborough.net, (650) 375-7422) to submit the ADU application; class 1 projects can go straight to Building for permits.
  4. Include your projected annual rent with the building permit application, and report actual rent each year after the unit is completed.

Common questions

Can I build an ADU in Hillsborough, California?
Yes. ADUs are permitted on Hillsborough residential lots with an ADU permit and building permit, approved without a hearing within 60 days of a complete application.
How big can an ADU be in Hillsborough, California?
A detached ADU up to 800 sq ft, 16 ft tall with 4-foot side and rear setbacks qualifies under the state-standard track. Larger units can be up to 850 sq ft, or 1,000 sq ft with 2 or more bedrooms, and must meet extra local design standards. A JADU can be up to 500 sq ft.
Do I have to live on the property to have an ADU in Hillsborough, California?
No. Owner occupancy is not required for an ADU. The minimum rental term is 30 days.
Does an ADU need parking in Hillsborough, California?
For larger (class 2) ADUs, 1 space per ADU or bedroom, whichever is less, with many exemptions.

California's statewide ADU law

California's ADU law (Gov. Code §§66310–66342, recodified from §65852.2; latest amendments effective January 1, 2026) overrides conflicting city rules, including in charter cities. Cities must approve ADUs ministerially (no hearing) within 60 days of a complete application, or the permit is deemed approved.

What cities cannot require or do:

  • Owner-occupancy for an ADU.
  • A minimum lot size.
  • Side or rear setbacks over 4 feet for a new ADU, or any setback for a converted existing structure.
  • Size, lot coverage or floor-area rules that block an ADU of at least 800 sq ft. Size caps can't be under 850 sq ft (1,000 for 2+ bedrooms).
  • Parking within half a mile walking distance of public transit, or replacement parking for a garage converted to an ADU.
  • Impact fees on ADUs of 750 sq ft or less.

By right on a single-family lot: one converted ADU plus one junior ADU (JADU, up to 500 sq ft inside the house), plus one new detached ADU (up to 800 sq ft, 16 ft tall). Owner-occupancy may be required only for a JADU that shares a bathroom.

Short-term rentals: cities may bar ADU rentals under 30 days, and must for by-right ADUs, JADUs and SB 9 units.

Coming January 1, 2027: AB 956 raises the by-right count to two new detached ADUs per single-family lot.

SB 9 (§§65852.21, 66411.7) allows two homes on a single-family lot, or a split into two lots of at least 1,200 sq ft (with a 3-year owner-occupancy affidavit).

State sources: Gov. Code §66314 (ADU ordinance standards) · Gov. Code §66315 (no owner-occupancy requirement) · Gov. Code §66317 (ministerial review, 60-day deadline) · Gov. Code §66321 (size, setback and height limits on local rules) · Gov. Code §66322 (parking exemptions) · Gov. Code §66323 (by-right ADUs and JADUs) · Gov. Code §66333 (junior ADUs) · Gov. Code §66311.5 (impact fees) · AB 956 (2026), Stats. 2026, ch. 791 · Gov. Code §65852.21 (SB 9 two-unit projects) · Gov. Code §66411.7 (SB 9 urban lot splits) · HCD, Accessory Dwelling Units (ADU Handbook, updated March 2026)

What the ordinance says

Exact text from Hillsborough's published ordinances. Section numbers are as printed in the source.

  1. Sec. 17.16.010(B) (use table), Municipal Code Title 17 Ch. 17.16 Uses Permitted (17.16.015 home business permit; 17.16.020 commercial uses incl. transitory lodging prohibited), p. 1
    A "P" in the table below indicates that a use is permitted in the district.
  2. Sec. 17.52.020(C)(1), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    Every ADU and JADU requires an ADU permit and a building permit. The city will review and approve permit applications in accordance with subsection C.3. below.
  3. Sec. 17.52.020(C)(3)(c), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    The town must approve or deny an application to create an ADU or JADU within sixty days from the date that the town receives a complete application.
  4. Sec. 17.52.020(D)(1)(b), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    One detached, new-construction ADU on a lot with a proposed or existing single-family dwelling, if the detached ADU satisfies each of the following limitations:
  5. Sec. 17.52.020(D)(1)(b)(1), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    The side- and rear-yard setbacks are at least four feet.
  6. Sec. 17.52.020(D)(1)(b)(2), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    The total floor area is eight hundred square feet of livable space or smaller.
  7. Sec. 17.52.020(E)(2)(a)(1), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    Except as otherwise provided [below], a detached ADU created on a lot with an existing or proposed single-family or multifamily dwelling unit may not exceed sixteen feet in height.
  8. Sec. 17.52.020(F)(1)(a), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    Eight hundred fifty square feet of livable interior space for a studio or one-bedroom unit; and
  9. Sec. 17.52.020(F)(1)(a)(2), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    One thousand square feet of interior livable space for any unit with two or more bedrooms.
  10. Sec. 17.52.020(E)(7)(a), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    ADUs. ADUs are not subject to an owner-occupancy requirement.
  11. Sec. 17.52.020(E)(4)(a), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    No ADU or JADU may be rented for a term that is shorter than thirty days. This prohibition applies regardless of when the ADU or JADU was created.
  12. Sec. 17.52.020(F)(6)(a), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    One off-street parking space is required for each ADU or bedroom, whichever is less. The parking space may be provided in setback areas or as tandem parking, as defined above.
  13. Sec. 17.52.020(F)(2)(b), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    Front. ADUs that are subject to this subsection F. must conform to twenty-foot front setbacks, notwithstanding subsection F.1.c. above.
  14. Sec. 17.52.020(F)(3), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    Notwithstanding subsection F.1.c. above, no ADU subject to this subsection F. may cause the total lot coverage of the lot to exceed 50 percent, or to exceed 40 percent in any setback area adjacent to a street line.
  15. Sec. 17.52.010 (JADU definition), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    It is no more than five hundred square feet of interior livable space in size.
  16. Sec. 17.52.020(G)(2)(a), Municipal Code Title 17 Ch. 17.52 Accessory Dwelling Units and Junior Accessory Dwelling Units, p. 1
    No impact fee is required for a JADU or for an ADU that is less than seven hundred fifty square feet of interior livable space.
This page summarizes Hillsborough's posted ordinances as of 2026-10-05. Ordinances are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Town of Hillsborough Code Enforcement (Code Compliance Coordinator), 1600 Floribunda Ave.; Building Division (650) 375-7411, Planning (650) 375-7422 ((650) 375-7572) before you build. Spot an error?