Piatt County, Illinois

Founding: 1841
Parent counties: DeWitt and Macon

Namesake: James Piatt, local resident
Seat: Monticello (1841–)

Land area: 439 square miles

Population (2020): 16,673

Population (historic): 1,606 (1850); 10,953 (1870); 17,062 (1890); 16,376 (1910); 15,588 (1930)

Periods of population growth: pre-1850–1900; 1950–1980; 1990–2010

Subdivisions: One city; six villages; two CDPs; eight townships

Pre-1940 residences (estimated): 1,950 (26.3%)
Pre-1940 housing survival rate: 53.3%
Pre-1860 housing survival rate (1940): 8%

Farm housing in disrepair (1950): 4.2%

Nonfarm housing in disrepair (1950): 6.3%

Average farmhouse value (1930): $2,537
Average outbuilding value (1930): $3,060

Average farmhouse size (1940): 6.6 rooms
Number of farms (1920): 1,386
Average farm size (1920): 182.5 acres
Sources of settlement: Ohio, Indiana, and Pennsylvania
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Monticello

Namesake: Monticello, Thomas Jefferson's plantation

Founding: 1837 (platting); 1837 (post office); 1841 (incorporation)

Population (2020): 5,941

Population (historic): 163 (1850); 871 (1870); 1,643 (1890); 1,981 (1910); 2,378 (1930)

Periods of population growth: pre-1850–1900; 1910–1980; 1990–2020
Pre-1940 residences (estimated): 590 (22.9%)
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Outstanding Buildings

1. William Wack House (c. 1875). 2907 North 1300 East. Blue Ridge. Italianate.
2. William Piatt Farm (c. 1865). 1889 Piatt Road. Goose Creek. Italianate.
3. Michael Harshbarger House (1876). 250 State Route 50. Unity. Italianate.
4. Voorhies Castle / Nels Larson Farm (1900–1904). 525 Voorhies Castle Court. Unity. Queen Anne/Classical Revival.
5. Robert Allerton Estate (1899–1900). 515 Old Timber Road. Willow Branch. Georgian Revival.

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