The building, itself, is in a traditional observatory design, Colonial Revival style, following a T-plan.
The dome rises 35 feet (11 m) in the air.
The observatory was built on a one-story T-plan, facing north, of buff-colored Roman brick (from Indiana) and features limestone lintels and sills.
The cross of the T is 75 feet (23 m) long east to west and 25 feet (7.6 m) deep, its stem is located to the south, centered along the east-west axis and is 26 feet (7.9 m) deep by 25 feet (7.6 m) wide.
The octagonal observation tower rises 25 feet (7.6 m) into the air at the intersection of the T where it becomes a dome and continues to a height of 35 feet (11 m).
At floor level of the second equatorial room a balustrade circles around the exterior of the tower.
The tower is capped by a great circular limestone plate, which carries the dome track.
Internally, the diameter of the dome is 24.5 feet (7.5 m) and its zenith 24 feet (7.3 m) above the floor.
The dome slit, which still operates, has an opening of 44 inches (1,100 mm) and can be opened or closed by hand in seconds.
The dome tower and equatorial room are original save a motor drive which replaced the old rope and sheave method of rotating the dome.
 As of September 2014, the motor drive is being serviced and the rope and sheave method is once again in use.

In the center of the equatorial room is the 1896 12-inch (300 mm) refractor telescope.
Built by the firm of Warner and Swasey, Cleveland, Ohio, the scope is stabilized on a brick pier which extends down into the bedrock and is not attached to the building in any way.
The telescope cost $4,500 and still has the original observer's chair mentioned in the contract with Bevis and Company at a cost of $25.
The entrance hall, below the equatorial room, octagonal in shape, is centered on a brick pier.
The entrance hall retains original stairs, newel posts, balustrades, and wood floors; it is still used for its original purpose, storage.

The east and west wings of the building once each contained a transit room.
Each of the rooms had a mounted transit telescope on a brick pier; the piers are still visible in the basement below the transit rooms.
The western transit rooms were converted into office space by the 1920s.
The eastern transit rooms were converted to office space more recently.

The exterior of the observatory building has a brick cornice, with stone sills and lintels, stone water course, ornamental gutters, and original copper downspouts.
Most of the building's windows are of the wooden double-hung variety and original, as are the front entrance door transom and concrete stoop.
The original front balustrade has been replaced, however, the western stoop and ornamental iron balustrade is still original.

Aside from the transit room conversion to office space the building has seen other major work in the past.
The southwest corner of the building was built in 1956, of cream colored brick, to house additional classrooms and office space.
The addition of 1956 took special care to replicate nearly every aspect of the original building except for color.
Another major addition occurred in 1966 with the construction of the large east wing.
Of the same cream colored brick as the 1956 addition, it also tried to mimic the building's finer details.
The 1966 east wing addition provided for, again, more office space, but this project also included space for a new darkroom and a radio telescope laboratory.
The observatory basement and the dome housing the refractor are still in use by the astronomy department at U of I and the University of Illinois Astronomical Society, a student organization on campus.
The building, itself, is in a traditional observatory design, Colonial Revival style, following a T-plan. The dome rises 35 feet (11 m) in the air. The observatory was built on a one-story T-plan, facing north, of buff-colored Roman brick (from Indiana) and features limestone lintels and sills. The cross of the T is 75 feet (23 m) long east to west and 25 feet (7.6 m) deep, its stem is located to the south, centered along the east-west axis and is 26 feet (7.9 m) deep by 25 feet (7.6 m) wide. The octagonal observation tower rises 25 feet (7.6 m) into the air at the intersection of the T where it becomes a dome and continues to a height of 35 feet (11 m). At floor level of the second equatorial room a balustrade circles around the exterior of the tower. The tower is capped by a great circular limestone plate, which carries the dome track. Internally, the diameter of the dome is 24.5 feet (7.5 m) and its zenith 24 feet (7.3 m) above the floor. The dome slit, which still operates, has an opening of 44 inches (1,100 mm) and can be opened or closed by hand in seconds. The dome tower and equatorial room are original save a motor drive which replaced the old rope and sheave method of rotating the dome.  As of September 2014, the motor drive is being serviced and the rope and sheave method is once again in use.
In the center of the equatorial room is the 1896 12-inch (300 mm) refractor telescope. Built by the firm of Warner and Swasey, Cleveland, Ohio, the scope is stabilized on a brick pier which extends down into the bedrock and is not attached to the building in any way. The telescope cost $4,500 and still has the original observer's chair mentioned in the contract with Bevis and Company at a cost of $25. The entrance hall, below the equatorial room, octagonal in shape, is centered on a brick pier. The entrance hall retains original stairs, newel posts, balustrades, and wood floors; it is still used for its original purpose, storage.
The east and west wings of the building once each contained a transit room. Each of the rooms had a mounted transit telescope on a brick pier; the piers are still visible in the basement below the transit rooms. The western transit rooms were converted into office space by the 1920s. The eastern transit rooms were converted to office space more recently.
The exterior of the observatory building has a brick cornice, with stone sills and lintels, stone water course, ornamental gutters, and original copper downspouts. Most of the building's windows are of the wooden double-hung variety and original, as are the front entrance door transom and concrete stoop. The original front balustrade has been replaced, however, the western stoop and ornamental iron balustrade is still original.
Aside from the transit room conversion to office space the building has seen other major work in the past. The southwest corner of the building was built in 1956, of cream colored brick, to house additional classrooms and office space. The addition of 1956 took special care to replicate nearly every aspect of the original building except for color. Another major addition occurred in 1966 with the construction of the large east wing. Of the same cream colored brick as the 1956 addition, it also tried to mimic the building's finer details. The 1966 east wing addition provided for, again, more office space, but this project also included space for a new darkroom and a radio telescope laboratory. The observatory basement and the dome housing the refractor are still in use by the astronomy department at U of I and the University of Illinois Astronomical Society, a student organization on campus.
Astronomical Observatory (University of Illinois at Urbana–Champaign)