Is it at all common for new employees to return marked-up agreements?

In the northeast part of the US, I have never seen this happen in over 35 years of work. I don't believe it is at all common here - practices may be different in your part of the world. (It's certainly a different case for contract work - as you indicate, every contract can be different) In companies where I have worked, the standard employment contract has been crafted by HR and company lawyers. In my companies, I'm confident that it would be difficult (and time consuming) to have changes made and have both parties come to an agreement.

Before just marking up such an agreement and handing it back, I'd suggest that you discuss the portions that concern you with either the HR person who gave you the agreement or with the hiring manager. You may find that modifying the agreement to suit your taste is common, permitted, or not allowed - any of these might be possible in this one company.

If you find that changes to the employment agreement are discouraged, you will then have the choice of insisting, or just letting it go - and either direction can be viewed amicably.

I'm a strong believer that it never hurts to ask (at least in my part of the work, perhaps not in yours), if you ask in a polite, non-confrontational way.

Good luck!

Is it at all common for new employees to return marked-up agreements?

In the northeast part of the US, I have never seen this happen in over 35 years of work. I don't believe it is at all common here - practices may be different in your part of the world. (It's certainly a different case for contract work - as you indicate, every contract can be different) In companies where I have worked, the standard employment contract has been crafted by HR and company lawyers. In my companies, I'm confident that it would be difficult (and time consuming) to have changes made and have both parties come to an agreement.

Before just marking up such an agreement and handing it back, I'd suggest that you discuss the portions that concern you with either the HR person who gave you the agreement or with the hiring manager. You may find that modifying the agreement to suit your taste is common, permitted, or not allowed - any of these might be possible in this one company.

If you find that changes to the employment agreement are discouraged, you will then have the choice of insisting, or just letting it go - and either direction can be viewed amicably.

I'm a strong believer that it never hurts to ask (at least in my part of the world, perhaps not in yours), if you ask in a polite, non-confrontational way.

Good luck!