After purchasing Noormarkku Works, Antti Ahlström wanted to build a home for his family and an office for his business in Noormarkku. As early as 1873, Anti Ahlström had inquired about the drawings of the main building of Noormarkku Manor from State agronomist Henry Gibson and county architect Heideken. However, none of their plans have survived. The final drawings were commissioned from architect Evert Lagerspetz (1847-1884) and the construction work was carried out in 1877, but the building with its valuable interior was not completed until the fall of 1881. The two-story log building is in the shape of an L, with a tower rising at its riverside end, featuring open observation decks. The tower’s top was renovated in the 1890s. Initially, the kitchen wing was single-story. Isotalo’s exceptionally large wooden structure reflects the richly ornate Neo-Renaissance style of its time.

The interior architecture follows the same style. Interior decoration involved Salomo Wuorio’s painting company in the 1890s (artists Chorin and Wetterstrand). Finnish cultural figures’ names are painted on the ceiling of the dining room. Double pilasters adorn the walls of the grand hall. Isotalo had a water pipe installed in 1882, and electric lighting was installed in 1896. Isotalo served as both the Ahlström family home and the company’s headquarters until 1916. The surrounding park was designed by both Henry Gibson and state pomologist Alexandra Smirnoff. At the edge of the lawn stands a playhouse of the same age as the main building, also in pure Neo-Renaissance style. The western fountain with its sculptures still remains, one of the two fountains located on either side of the building.

The building was restored in 1967 according to the plans of architect Marius af Schulten. Isotalo is one of the few wooden residential buildings designed by architect Evert Lagerspetz. Lagerspetz, who designed several barracks buildings, died in 1884.