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

National Register listings: Six

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.