The costs in the base case for new build and fit-out, of £3,377 per square metre, were based on standard office accommodation, except those for the computer hall, which were higher to reflect the more specialised nature of that accommodation. Estimates from Northcroft reflected that GCHQ's specialised requirements could suggest costs of around £4,000 per square metre. This would add some £42 million to the public sector comparator costs.
Refurbishment costs could similarly be increased, giving an additional £14 million.
Capita consultants advised that design risk may have been underpriced in the base case. As a result, GCHQ added a further 10 per cent as a test. This gave an additional £4 million.
The model assumed that construction started in year 1. If this were brought forward to year 0, this would lead to an increase in the Net Present Value to reflect a year's less discounting. However, this would be offset against a reduction in the tender price inflation figure. GCHQ estimated the net effect of this would be an increase of £4 million.