Delays on projects are very common.
Wet weather, delays in client review of designs, transport delays due to strikes, and lots of other things can cause delays that are outside of your control.
However, your client will most likely not want to pay for these delays or will not want to grant extensions of time for these delays.
In fact, some contracts will require a very specific form of notification to the client for you to be able to claim extra costs or extra time on the project.
Make sure you record all delays, their reasons, the associated costs, how long the delay extends the schedule, and who caused it (or what caused it).
Also, make sure you inform the client as soon as possible about the delay. Update the schedule and send them the update as well, highlighting the expected extra time the project will take due to these delays. You should also make sure you use the correct method for notifying the client of the delay (as per the contract). This may be in the form of a formal extension of time notification on a specific template or similar.
Many contracts will have a set period allowed for notification and application for extension of time. If you do not notify the client (and apply for the associated extension of time) within that allowed period, you will most likely not be eligible for an extension of time, and the client may require you to still meet the previously agreed schedule.
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If your client increases the project scope, or asks your project team to do something extra outside of your agreed scope, ensure you inform the client of the effects on the project schedule.
Notify your client of delays in the project schedule as soon as possible, and especially within the contractually allowed period of time. If you don’t, the client may hold you to the previously agreed schedule, even if the delay is their fault.