Although most of the base structure materials that are used in building can withstand some wet weather during the build process, it is important that the integrity of these materials is monitored and checked prior to any linings or finishing works being applied.
Framing materials, brickwork, sheet flooring, concrete etc are all regularly exposed to weather early in the process. All of these materials are either treated in some way to resist moisture or robust enough to withstand some weather. (Other materials like bulk insulation are not and should be protected).
However, although these materials are robust and treated, they are not immune to damage or processes which can occur due to the effects of moisture. By example, we have discovered some moss growth on some of the existing brick walls that we are retaining that are not getting a lot of natural light at present and have been soaked from recent weather. This is not an unusual occurrence, particularly with older structures, but it is something that needs to be treated.
Mould, moss etc needs to be completely removed prior to lining and there are applied products that will do this. The last thing you want is interstitial mould/ moss growing in your wall cavities.
The same advice goes for wet framing, insulation etc. All of this structure needs time to dry out before lining. If not, apart from potential moisture retention issues and the problems that can cause, you can also have issues with expansion and contraction of framework meaning fixings will not take and can pop back through plasterboard linings as an example.
In summary, wet weather can be sustained, but make sure your contractor ensures that this has all had time to dry, and any surface growth such as mould/moss is removed in full before anything is enclosed.
Image to the left of the rear addition footprint now taking shape.