Estimated contract duration

Hello, I would like to get some advice on how best to manage a contract where the duration cannot be easily determined. We are working on a large civils site where sitewide scaffolding will be required for a duration that could range from 2 to 5years. NEC3, I see several options:

  1. Price as a lump sum (Option A) for the 2years. Once this point is reached the PM instructs an extension via a rolling PMI. This could be a monthly PMI, or longer durations until Completion is reached. Each extension in time forming a CE.
  2. Use an Option B, estimate the most likely duration for the whole of the works, remeasure as you go. Would likely need X1 included. The scaffold items listed as individual line items, quantity x rate, with the quantity being time related i.e. “independent scaffold price per week, per M2” = XXX.
  3. Use Option A, PMI issued at end of year 2. The PMI has a set of Project Managers Assumptions (time related), at Completion the assumptions are corrected using cl.60.1.(17). Allows an accurate price to be achieved at Completion.
  4. Use a Term Service Contract

All views, or alternatives welcomed.

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Any suggestions/advice much appreciated.

Any answer appreciated.

@engineering I would personally avoid rolling PMIs and extensions as they are a recipe for disputes arising out of quotations etc.

If you take the route of Option A with the max. duration (e.g. 5 years) you can always omit scope under a “negative” CE, if the scaffolding is no longer required.

I think that option will be cheaper and less troublesome but in the long run.