新築 and 改築: new building and changes to an existing building
Compare a newly built house and library with the conversion of an old barn and renovation of a school building. Follow new construction versus alteration of an existing building.
New construction and a newly built state
新築 (しんちく) means new construction or newly built. 「駅の近くに家を新築した。」 means “We built a new house near the station.” 新築した describes the past act of building the new house.
「新築の図書館は木の香りがする。」 means “The newly built library smells of wood.” 新築の modifies a library that is newly built. The sentence describes its present smell of wood, rather than construction still in progress.
Changes to existing buildings with 改築
改築 (かいちく) means renovation or rebuilding. 「古い納屋を住居に改築した。」 means “We converted the old barn into a home.” The old barn is an existing building converted into a home.
「校舎の改築工事が春から始まる。」 means “Renovation work on the school building will begin in the spring.” This example names renovation work on an existing school building, beginning in the spring.
The relationship to the building
The central comparison is new building with 新築 versus changes to an existing building with 改築. The difference does not simply arise because one example mentions a house and another a barn.
Keep the past action in 新築した separate from the newly built state in 新築の. Likewise, the barn conversion and the school renovation beginning in spring have their own stated timelines.
What do 新築の図書館 and 古い納屋を住居に改築した describe?
Show the answer and why
The first describes the state of a newly built library. The second describes converting an existing old barn into a home. Their focus differs as new building versus changing an existing building.