With refurbishments and the reuse of existing buildings, usually the items that are worth ‘modeling’ would be items that would last the lifetime of the building (i.e. structural components) that won’t get replaced within the study period. This is because for the items that will get replaced within your study period, while they were retained from the original building, you will still have to account for the impacts of them getting maintained/replaced/repaired within the study period of the building. Although we encourage the reuse to existing materials, however to model this correctly would not only be complicated but ultimately the GWP savings from that would not really be worth the effort.
For example, retaining plasterboard linings which have an assumed service life of about 45 years, would need to be replaced twice in a building study life of 100 years. So you would still need to account for the impact of the new plasterboard linings that was installed in later in the life of the building.
I’m a bit confused about the GFA of your final building – is the final completed building going to be 90,000m2 or 65,000m2 or 155,000m2?
For example, if your final completed building will be 90,000m2 but 60,000m2 is retained from the existing building. In your Reference Design, you model the whole 90,000m2 as a brand-new building and in your Proposed Design ‘scenarios’ you delete 60,000m2 of the structural components of the building to demonstrate the benefit of reusing the old building.
In a different example, if your final completed building is 90,000m2 but your existing building is 100,000m2 and only 60,000m2 of the existing building will be retained. Which means that 40,000m2 of the existing building was demolished. You will still model it the same way as the above because the demolition of the 40,000m2 of existing building is out of scope for the new building’s study according to EN 15978.
Hope that answers your question.