We always get nervous when the Contractor or the Client rings asking to change this or that. This would typically happen at construction stage and after we have spent long hours preparing construction drawings, specifications, etc. Many details are quite complex and it is not easy to change them from A to B in the hour. Furthermore, they are likely to involve extensive calculations to comply with Part L of the Building Regulations and possibly more research.
It would be easy enough to change the colour paint for a bedroom but changing a whole wall construction or even just the wall insulation is not that easy, it is time consuming and it kind of defeats the purpose of preparing pre-construction drawings.
Recently we found out on site that an existing wall was not as we had assumed in our drawings and calculations. We were prepared to review the specification and there were notes in our drawings that requested the Contractor to confirm the type of wall before proceeding with works. This was not a change requested by the client or contractor, it is just an example of what is involved when something changes. Between research, new drawings, calculations, etc there were about 10 hours extra to properly detail this variation. The image below shows one of the U-value calculations for just ONE wall. The client rarely gets to see all the work that is involved when things change on site.
In conclusion, unless there is a really good reason to change the specification, please think careful before you ask and please understand the implications not only for your Architect but also for the Assigned Certifier.
And even more important – please do not change the specifications without getting permission from the Architect/Contract Administrator!