Preassembly
Pre is a prefix meaning before, so to preassembly or pre-assembly refers to assembling something before or in advance.
In construction terms PAS 1192-2:2013: Specification for information management for the capital/delivery phase of construction projects using building information modelling (BIM), defines assembly as: 'Group of components or types to enable the reuse of standardised design or specification elements improving productivity of design and delivery as well as providing a location to hold specifications and lessons learnt in a simple and useable way. They may hold benchmark data for cost and carbon impacts. The contents of assemblies may themselves have attributes and classifications. These properties may include key data which is attached (to the object) for use once it is placed into a model and may include cost, CO2, programme, maintenance and other key information.'
So pre-assembly in design and construction terms refers to elements of a building that have been assembled in advance, prior to being installed in the building. Some common examples of this might be where a bathroom unit might be pre-assembled with sanitary units already fitted and plumbed to be lifted into place by crane, or similarly, a group of structural members such as a roof truss that is pre-assembled and lifted into place on a roof.
Unassembled would refer to the prepared parts that have not yet been put together, unassembly per se is not a word.
Offsite residential construction, Glossary of terms, published by Buildoffsite in 2018, defines pre-assembly as: ‘The manufacture and assembly of a complex unit comprising several components before the unit is installed onsite. Offsite is a commonly-used term for construction of this type.’
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