A Design Contingency is included in a Cost Plan for minor changes during the development of the design and documents. It should be anticipated in cost plans that items may change or be added (by necessity or by request).
A Design Contingency is also for items yet to be confirmed or considered, or, where there is little or no information during design… for example: more joinery; a new basin; extra structural framing; a more expensive floor finish; unforeseen repairs etc…
As the quality and content of the documents improve, the amount required or % of the design contingency should decrease. At the time of tendering a cost plan is unlikely to include a Design Contingency allowance as all items of design should be considered and documented as needed. Any unknown costs or items at this time should then be dealt with using PC Sums, Provisional Sums and Provisional Quantities.