After the Jämsänkoski factories became part of United Paper Mills Ltd, a period of brisk building began. The pulp factory was enlarged and acid towers rose up on the landscape, first a wooden one and later one built of concrete. Several bridges were built across Hovilanhaara for internal works traffic. In 1934 stylish white woodchip silos were completed, constructed using the slip-casting method for the first time in Jämsänkoski. In the 1930s, air conveyors, transporting woodchip and other raw materials from Alaja harbour to the factory site, appeared on the landscape. The harbour area had grown after the sawmill moved there at the end of the 1920s. The timber stores moved away from the factory site when the debarking plant was moved to the river bank, about half a kilometre from the factories.

The production area was separated from the surrounding habitation when the whole of the factory site was fenced off in 1932. Entry to the factory site was through guarded gates. At Jämsänkoski, the fence also symbolised the final move to the era of large-scale industry. The old Patalankoski paper mill, which a few years earlier had been converted into an assembly hall, burned down and was demolished in 1936. In its place was built Ilveslinna, the company social club. The east bank of the rapids was a kind of transitional area from the factory to the residential area.

Over the decades, the factory site changed ever more distinctly into a specialised production area. Containers, conveyors and various structures required by industrial activities rose up on the landscape. The rapids channel changed when the Patalankoski power station water intake pipe was built in the place of the former log channel. Rukkosaaret islands at Koskikeskinen were submerged as the water level was raised. The building of the factory's water treatment plant at the end of the 1940s finally closed Hovilanhaara.

In the following decades the factory site expanded to the west and to the south. New papermaking machines demanded ever larger buildings. The completion of the railway and the building of Olkkola sawmill quietened the Alaja area. The railway bridge over the river was constructed to the south of the factory site. At the beginning of the 1960s, a new acid tower was completed and became a landmark of the factory site. In aerial pictures the red brick core of the factory site can still clearly be seen. At the end of the decade, a new evaporator and incinerator plant for spent liquor was built on the south side of the old paper mill. The salle for the fourth papermaking machine was erected by the Jämsä road and a new gate house, with a tunnel leading to the factory, was built on the east side of the road in 1971.

In the 1970s, the Pekilo factory, which was more a plant than a building, was constructed on the west side of the factory site. The factory site had already expanded southwards about a kilometre from the old factories. The furthest away was the reception and storage area for timber.

The biggest change to the landscape arrived in the 1980s, when the Jämsänjoki river, flowing through the factory site, was redirected into a 640 metres long underground tunnel to accommodate the construction of the PK 5 salle. The new salles demanded by new papermaking machines and the new mechanical pulp refinery rise above the rest of the mass of buildings. The difference in the scale of the factory area and the surrounding residential area has grown during each decade of operation of the Jämsänkoski factories.