What's Required in Philadelphia
- Permitted only in a historic structure, or in the RSA-5 or CMX-1 districts where the principal house is detached or semi-detached and the lot is at least 1,600 sq ft with no more than one unit otherwise permitted (Zoning 14-604(11); ADU checklist)
- Council Districts 1, 2, 3, 7: historic or qualifying lots; Districts 4, 5, 8, 9: historic structures only; Districts 6, 10: not permitted (Zoning Quick Guide, Feb 2026)
- ADU must be within the principal building or an existing detached accessory building such as a garage; no new detached ADUs
- Maximum floor area 800 sq ft unless within a historic structure; one ADU per lot; only one street-facing entrance unless one already existed
- Owner must occupy the main house or the ADU and record an affidavit and deed restriction before final occupancy
- Zoning/use registration permit ($189 plus $25 filing applied) and a building alteration permit with plans ($76 first 500 sq ft plus $56 per 100 sq ft) reviewed within 15 business days
- Plumbing permit by a licensed master plumber ($50 first 7 fixtures), electrical permit by a licensed electrical contractor ($25 per $1,000, $63 minimum), mechanical permit by a licensed contractor ($69 appliance or $189 ductwork), each plus $7.50 surcharges
- 2021 Residential Code interior standards apply: 7 ft ceilings, habitable rooms of 70 sq ft with 8% window area, interconnected smoke alarms, compliant basement egress
Can You Build an ADU in Philadelphia?
Only in narrow circumstances, and never as a new detached building. Zoning Code Section 14-604(11), per L&I's ADU checklist (PZ_004) and the February 2026 Zoning Quick Guide, permits an ADU in two situations. The first is inside a historic structure: a building designated by the Historical Commission or contributing to a designated district. The second is in the RSA-5 or CMX-1 zoning districts, the rowhouse districts, where the principal house is detached or semi-detached, the lot is at least 1,600 square feet, and no more than one dwelling unit is otherwise permitted. Geography filters both: the Quick Guide allows ADUs in historic structures or on qualifying lots in Council Districts 1, 2, 3 and 7, in historic structures only in Districts 4, 5, 8 and 9, and not at all in Districts 6 and 10 (the checklist uses the /FNE, /NE, /FDO, /VDO, /NDO and /EDO overlays). A bill to loosen these rules (250042) was introduced in January 2025 and was still only introduced on L&I's June 2026 legislation table.
Philadelphia ADU Rules by Scenario
| What you are doing | City filing | Rule |
|---|---|---|
| Basement or rear-wing apartment in a semi-detached RSA-5 house, lot 1,600 sq ft or more, Council District 1, 2, 3 or 7 | Zoning/use permit, building permit, plumbing, electrical, mechanical permits | ADU checklist steps 2-3; Quick Guide |
| Converting an existing detached garage into an ADU on a qualifying lot | Same permit stack; garage must predate the 2012 Zoning Code | Checklist: existing detached accessory building |
| New detached backyard cottage | Not permitted by zoning | Checklist: ADU must be within an existing building |
| ADU in a designated historic house, any residential district, Districts 1-5, 7-9 | Permitted; floor area may exceed 800 sq ft | Checklist Step 3; Quick Guide |
| Attached rowhouse in RSA-5 that is not historic | Not permitted (must be detached or semi-detached) | Checklist flowchart question 5 |
| Any ADU in Council District 6 or 10, or a non-historic ADU in 4, 5, 8, 9 | Not permitted; ZBA variance is the only route | Quick Guide; zoning page on variances |
| Second ADU on the same lot, or an ADU in a non-owner-occupied house | Not permitted | Checklist: one ADU per lot; owner occupancy |
How Much Does an ADU Permit Cost in Philadelphia?
A permitted ADU is an interior conversion, priced from L&I's ordinary fee rows. The zoning fee summary lists $189 per use for use registration, which the accessory-uses FAQ says an ADU needs; the $25 zoning filing fee is applied to it. The building permit is an alteration: $76 for the first 500 square feet plus $56 per additional 100, so a 600-square-foot conversion is $132 and the 800-square-foot maximum is $244, each plus the $3 city and $4.50 state surcharges. Plumbing for the new kitchen and bath is $50 for the first 7 fixtures; electrical is $25 per $1,000 of electrical cost with a $63 minimum; mechanical is $189 for ductwork with appliances or $69 for a stand-alone appliance. A small 600-square-foot conversion at the electrical minimum with a ductless unit totals $139.50 + $189 + $57.50 + $70.50 + $76.50 = $533. The 800-square-foot unit with $6,000 of electrical and ducted HVAC totals $251.50 + $189 + $57.50 + $157.50 + $196.50 = $852. A certificate of occupancy carries no additional fee as part of the building permit process.
Conditions Every Philadelphia ADU Must Meet
The checklist's Step 3 lists them. The owner must occupy either the main house or the ADU, and before final occupancy must record an affidavit and deed restriction promising to keep living on the property. Only one ADU is allowed per lot; it may not exceed 800 square feet unless it is in a historic structure; only one entrance may face the street unless a second already existed; and it must sit within the principal building or a detached accessory building, such as a garage, that existed when the Zoning Code took effect. Inside, the 2021 Residential Code applies; L&I's EZ Alterations standard shows the baseline: 7-foot ceilings, habitable rooms of at least 70 square feet with window area of 8% of the floor, interconnected smoke alarms in every bedroom and on every story, and no basement sleeping units without compliant ceiling height and egress.
Who Files and How Long It Takes
An ADU is never an EZ permit: the building application needs plans and is reviewed within 15 business days for a one- or two-family dwelling, and L&I can review zoning and building together for small residential projects. A licensed master plumber pulls the plumbing permit, a licensed electrical contractor the electrical permit (inspected by a licensed electrical inspection agency), and a licensed contractor the mechanical permit. A homeowner may hold a building permit only for work that needs no electrical or plumbing permit, which rules out an owner-built ADU. If the lot fails the checklist, the only path is a zoning refusal appealed to the Zoning Board of Adjustment.
How to Get a ADU Permit in Philadelphia
Run the ADU checklist
On Atlas confirm the base district (RSA-5 or CMX-1), overlays, Council District, lot area of 1,600 sq ft or more, and whether the house is detached or semi-detached or the building is historic.
Design within the limits
Keep the unit inside the existing house or garage, at or under 800 sq ft, with one street-facing entrance, and meet 2021 Residential Code ceiling, room, egress and alarm rules.
File zoning and building together in eCLIPSE
Submit the use registration and building permit applications with plans and the $25 filing fees; L&I reviews small residential projects jointly within 15 business days.
Have the trades pull their permits
A licensed master plumber, a licensed electrical contractor and a licensed mechanical contractor each file their permit after the building permit issues; you hire a licensed electrical inspection agency.
Record the deed restriction and get the certificate
Before final occupancy record the owner-occupancy affidavit and deed restriction, pass final inspections, and receive the certificate of occupancy included with the building permit.