If its an interior door I would cut a piece of wood in the 35 mm range and thinner than the door. Then nail or glue it to the bottom of the door. Being thinner the obvious seam will not be too annoying. If you need even less gap I would do the same then add a brush type door sweep.
This is the exterior door, I hope? You could remove the door and add more material to the bottom (wood). You could switch out the door with a larger one. You could add some trim below the door, but then that’ll be where people walk, and 20mm is probably too much unless you change the floor heights or add a ramp.
Interior doors have gaps at the bottom to allow air to circulate back to central furnace/AC. If there isn't a return air grill you will reduce flow into the room by closing the gap.
Your submission has been removed for one or more of the following reason(s):
You're asking what product or tool to buy.
You're asking about the features, properties or capabilities of a product or material made by someone else.
You're asking where to buy a product or tool.
You're asking for suggestions for a product or tool you intend to buy.
We do not permit posts asking for suggestions on materials and tools. This is because of spam.
/r/DIY is focused on the how, what technique to employ and which tool to use.
If you need recommendations for a product, you'll probably have better luck in a more specialised subreddit, e.g. /r/HomeImprovement, /r/woodworking, r/helpmefind, etc.
JonJackjon t1_j671zet wrote
If its an interior door I would cut a piece of wood in the 35 mm range and thinner than the door. Then nail or glue it to the bottom of the door. Being thinner the obvious seam will not be too annoying. If you need even less gap I would do the same then add a brush type door sweep.