Hard Constraint (Mandatory Constraint)

A hard constraint — also called a mandatory constraint — is a date imposed on an activity that the scheduling tool will honour even if the network logic calculates a different date. Common types include Must Start On, Must Finish On, Start No Earlier Than beyond the data date, and Finish No Later Than. Hard constraints override the forward and backward pass calculations, which means they can mask negative float, create artificial critical paths, and make what-if analysis unreliable. DCMA 14 allows fewer than 5% of activities to have hard constraints, though even that threshold is a guideline rather than an absolute.

Hard constraints are legitimate in specific circumstances: contractual completion dates, externally imposed start dates (such as planning permission conditions), interface dates locked in with third parties, and phased handover milestones that have been agreed with the client. These are genuine constraints that the network cannot override — the project must meet them. The problem is when constraints are applied not to reflect external reality but to make the programme look achievable, to force a target date, or to prevent the schedule from calculating negative float that would be difficult to explain.

The test for any hard constraint should be: is there a real-world reason this activity must start or finish on this date, and can we document it? If the answer is yes — planning condition, contractual milestone, statutory hold period — the constraint is appropriate. If the answer is 'we applied it because the director wants the project to finish in June,' it is a governance failure dressed as scheduling logic. Reviewers should ask contractors to provide the contractual or technical basis for every hard constraint in a programme. Unjustified constraints should be removed and replaced with the correct finish date on the relevant milestone activity.