When you need to adjust a project timeline because you need more time.

Overview

A short message with a professional tone will help offset any fears the client may have that you have not set a realistic timeframe.

Hi <Client>,

I wanted to make you aware of something important as soon as possible.

After working on <insert deliverable>, it's become apparent that to do my best work, I'm going to need until <insert date> to complete this.

I know this pushes our original deadline of <insert original deadline>, but rather than rush, I want to deliver an exceptional end result.

Can we agree to alter the delivery date to <insert date>?

<You>

Don't go into detail here—it's best not to act defensively.
Emphasize it's in the client's best interest to alter the timeline.
Keep bringing it back to how the client benefits.
Never say, "I need more time." Or "I miscalculated."
You want to maintain your authority and make the client feel like they're in good hands and the project will come off as planned.