Can I just plant a 1L grass into an existing patch of bog standard grass?
Yes. But the bog standard grass may well take over your new grass. (I'm assuming it's a decorative grass?)
I would probably dig a boarder around it and then put cardboard and woodchip down to slot the existing grass encroaching.
@hangedup started
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Can I just plant a 1L grass into an existing patch of bog standard grass?