I always find it frustrating that additional requirements and changes are added and the deadline is not extended

You should preface the start of every project with this.

"Every protect is different, but every project has the same problem, scope change. This is why for every additional change to the scope of this project after the first week we must extend the deliverable date by three days minimum. These days will be used in the following ways. 1) discovery & documentation 2) review and signoff 3) development. If at any time the delivery date is untenable further discussions should be had covering mvp status of features."

@sariel I usually go with the line that they either extend the deadline or remove something else.

Either way its always the team that decides how much will fit in the time allocated, management decides what should be there.