After Perry Ellwood inherited the home he remodeled the interior and exterior, drastically altering the home's appearance. Thus, the Ellwood House incorporates elements from several architectural styles. The site contains four buildings in addition to the main house. A 50-foot (15 m) tall water tower dominates the west side of the property while a 14-foot (4.3-m) tall miniature Stick style house is located nearer the main house. There is also a carriage house, which houses a visitor's center and a museum house that was once used to hold Harriet Ellwood's (Isaac's wife) collection of "curiosities." In 1964 the home was donated to the city of DeKalb and converted into a museum. The house was listed on the U.S. National Register of Historic Places in 1975.
Construction on the original mansion began in April 1879, George O. Garnsey, a Chicago architect, designed the Ellwood Mansion for DeKalb barbed wire entrepreneur Isaac Ellwood. At the time, Garnsey had designed other structures in DeKalb and Sycamore. By November 1879 the Ellwood family occupied the home. Newspaper accounts of the day put the cost somewhere between US$40,000 and $50,000. The original Ellwood House had a number of elements common to Victorian designed homes and combined several styles. Its mansard roof remains one of the home's most striking features. In addition, the home still incorporates Gothic columns, pitched gables, and a cast iron roof cresting with a trefoil design.
While Isaac Ellwood lived in the home large dinner parties, popular during the 19th century, were commonplace. The Ellwood House hosted prominent visitors throughout Isaac Ellwood's residence there. Theodore Roosevelt dined in the Ellwood House dining room while he was a candidate for Vice President of the United States in 1900. The dining room has also hosted U.S. Senators and U.S. state governors among other notable guests. Dinner parties at the Ellwood House followed customs typical to 19th century dinner partie. Guests "dressed" for the party, followed proper etiquette and were expected to know how to use the silverware properly.
The first remodeling work on the Ellwood Mansion took place as the 19th century drew to a close. The changes, commissioned by Isaac Ellwood, were meant to reflect more popular architectural styles including the Georgian and Colonial Revival. Many of the building's original Gothic features were replaced with more Classical elements. On the home's exterior, some of these changes can be observed in the addition of the portico and porte-cochere. Inside the home, the dining room was enlarged with the addition of the semicircular bay on the mansion's north facade.
Perry Ellwood inherited the house in 1910, and, together with his wife, May Ellwood (née Gurler), altered the mansion once again. The biggest changes were the addition of the terrace on the home's south face, a sunroom wing, and the relocation of the porte-cochere to the north side of the portico. This is the mansion that is visible today.