Hi @annaluno,
Thank you for your question about a drain under your deck.
It appears that this drain is purely there to collect surface water and funnel it elsewhere. If this is the case, it should be ok to build over, but there are some things you should consider.
My main point of consideration with building a deck over this drain is that you would limit your ability to get to the drain to clear it if needed. if the grate becomes covered, water will not be able to drain and will pool, creating a moist environment that will lower the life of your deck. If you didn't want to move the drain, you would need to install some form of access hatch to allow it to be cleared.
My suggestion would be to have a plumber relocate the drain to an area that is accessible with the installation of your new deck. This means you will have easy access to the drain if clearing is required.
Let me know if you have any further questions.
Jacob
