County · code 42003
Allegheny CountyPennsylvania
Allegheny County is in Pennsylvania with its seat in Pittsburgh. The county was formed on September 24, 1788, from Northumberland, Washington, and Westmoreland counties, and named after the Allegheny River.
- Founded1788
- County seatPittsburgh
- Named forAllegheny River
- Land area730 sq mi + 14 sq mi water
- Population1,250,578 2020 Census
- Parcels mapped584,511 checked 15 October 2026
Where it is
Allegheny on the map of Pennsylvania
Pennsylvania has 67 counties and county equivalents, each with its own records office. Shapes with a page are links.
History
Formed in 1788 from Northumberland County, Washington County and Westmoreland County
An old deed may name a county that no longer exists, or a boundary that has since moved.
Allegheny County was created from Northumberland, Washington, and Westmoreland on September 24, 1788. In 1792, the county gained the Erie Triangle, which Pennsylvania purchased from the federal government. From 1800 to 1803, Allegheny lost territory through the creation of Armstrong, Beaver, Butler, Crawford, Erie, Mercer, Venango, and Warren counties, though Beaver and Butler were briefly reattached before being detached again in 1803.
- 1788Allegheny County created Allegheny created from Northumberland, Washington, and Westmoreland. · Pa. Stat., ch. 1359, sec. 1/13:84-85
- 1789Gained territory Allegheny gained from Washington. · Pa. Stat., ch. 1435, sec. 1/13:319-320
- 1792Gained territory Allegheny gained all of the Erie Triangle, purchased by Pennsylvania from the federal government on 3 Mar 1792. · Pa. Stat., ch. 1624, sec. 13/14:238-239
- 1800Lost territory Allegheny lost to the creation of Armstrong, Beaver, Butler, Crawford, Erie, Mercer, Venango, and Warren, lost small area to Lycoming; Beaver and Butler attached to Allegheny. · Pa. Stat., ch. 2130, pp. 454-466
- 1803
- 1807Changed Middle of Monongahela River fixed as boundary of adjoining counties, including Allegheny (no change). · Pa. Stat., ch. 2863, sec. 1/18:644
Land and parcels
What the maps cover in Allegheny
Who publishes each layer, how good it is, and when we last checked.
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Parcels Good enough
Lot lines and tax-map numbers for 584,511 parcels from Allegheny County. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
edited 5 October 2026 · checked 15 October 2026 -
Address points Missing
No public address points layer found for Allegheny County yet.
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Buildings Reference
Roof outlines traced from aerial imagery from FEMA Response Geospatial Office with Oak Ridge National Laboratory (ORNL). Good for seeing what is on a lot; not accurate enough to set a fence from.
checked 18 October 2026
- Official
- from the office that keeps the record
- Good enough
- a reliable statewide or national copy
- Fallback
- a guide only; check the official record
- Missing
- nothing public found yet
People and land
Allegheny by the Census
Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.
Rules and offices
Who to ask before you build, dig or sell
Official county pages only. We link to them; we do not copy them, because they change.
Inside a city or town, permits usually come from that city rather than the county.
Permits
Zoning and code
County government
- County government
The official website of Allegheny County.
See your own lot on these maps
Open Allegheny County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.
Questions
Common questions about Allegheny County
- When was Allegheny County founded?
- Allegheny County was founded in 1788, formed from Northumberland County, Washington County and Westmoreland County, and named after Allegheny River.
- What is the county seat of Allegheny County?
- The county seat of Allegheny County is Pittsburgh.
- Is there an online parcel map for Allegheny County?
- Yes. Allegheny County publishes a parcel map of 584,511 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
- Has Allegheny County's boundary or name changed?
- Allegheny County was created in 1788 from Northumberland County, Washington County and Westmoreland County. Since then the Atlas of Historical County Boundaries records 4 later changes, chiefly involving Beaver County, Butler County and Armstrong County.
- Who issues building permits in Allegheny County?
- The county's page on building permits: Permits and Licenses - Allegheny County, PA. Inside a city or town, ask that city first.
- Who issues septic and well permits in Allegheny County?
- The county's page on septic systems and wells: Letter to Contactors (PDF, 98KB).
- Is my lot in a flood zone?
- We do not have flood maps for Allegheny County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.
Nearby