[DTV]

Apart from the technical upgrades, why does it take so long to build a new set?

Well it really it depends on the set. Twenty years ago when sets were just a couple of flats, a desk and a rear projection screen, you could install them in a weekend because it was just a matter of placing the pre-built items in the studio correctly. These days you have a lot of components that require extra stages - videowalls or multi-screen set ups require supporting frames to be installed and need to be calibrated to ensure a seemless image; colour-change LED lighting also requires calibration and often has to be intricately arranged within lightboxes.

The longer construction time is often compounded by, as you say, coinciding set builds with technical upgrades, but also it has become more common to undertake extensive pilots with new sets to ensure everything is working correctly and presenters are familiar with any new presenting styles.