The cost for a custom build shed of 16 feet x 6 feet (4.88 x 1.83m) is around £1,500. This is smaller than the available space, but biggest available. I am aiming for 5.18 x 1.98m. The £1,500 will get me a shed, I am after a timber building - with insulation and other nice things, but still within a tight budget.
One way to reduce costs is to hit eBay
eBay: Windows 1800x900 £25
eBay: Oak Door £10
eBay: Work Light £3