On buildings up to 30 sq meters we use a 75mm x50mm framework which is pressure treated at no more than 600mm centres.
On building over 30sq meters building regulations generally applies. Therefore we either build off plan or generally use 100mm x 50mm pressure treated timbers at 450mm centres with a centre spa. This adds 15% to the building cost.
On playdens and octagonal buildings we do not use a nail and frame system, but we end fix a 44mm board. These buildings are generally not pressure treated.