Compare prefabrication and site construction across the complete project: design choices, delivery access, foundations, connections and finishes. Prefabrication describes where components are made. It does not automatically establish quality, a complete delivery scope or the overall project duration.
Which elements are manufactured in advance?
A supplier may mean individual manufactured components, wall panels or a larger assembled unit. Ask which operations happen in the workshop and which remain on site. Define the delivery boundary precisely.
Foundations, the connection to the house, services and finishing may sit outside the quoted supply package. Prefabricated also does not identify the construction material by itself. Review the actual floor, wall and roof build-ups before treating two offers as equivalent options for the same extension or comparing their headline prices.
Do the logistics fit your property?
The dimensions and weight of assembled components influence transport and positioning. Discuss the route to the rear garden, available setup space and surrounding properties. Site construction also requires deliveries and storage, but potentially in different sizes and sequences.
A short lifting operation does not mean the site is needed only briefly. Ask what must be prepared before delivery and who verifies that dimensions and connection points match the manufactured element. The chosen method should respond to real access conditions rather than an ideal demonstration site.
Which design choices remain available?
A production system may offer a clear set of dimensions and details. That can be useful when they suit the intended room. An unusual plot or junction may need additional development. Site construction is not infinitely flexible either: structure, materials and buildability still matter.
Compare the same internal layout and finish, including the connection to the original house. Establish when changes remain possible and what happens after the design is released for production. This places flexibility at the relevant stage of the process.
How do you compare the full programme and responsibility?
Put design preparation, manufacturing, site work and handover on the same timeline. A short installation phase says little about the preceding period. Agree who establishes measurements and who manages the boundary between the supplied element and existing house.
Ask how concealed junctions are checked before finishes cover them. Choose the approach that suits the developed proposal and location. Without comparable scopes, general claims that one method is always cheaper, faster or better provide little useful information about your particular extension.