Another great and minimal design from Reiss, elegant yet could be wear casually with jeans and denim jacket under. Reiss unfortunately rarely produces slim fit coats or jackets so I was quite lucky to acquire this coat a year ago. Small modification made: I have to shorten the sleeves and I add neck strap to the collar as I always wear this jacket with collar up.
If you want to wear your overcoat or elegant coat casually then try to add layers below the coat like denim jacket or cardigans with t-shirt and contrast it with skinny jeans, casual boots or smart trainers.
Pea coats are my one of my favorite jackets as they are very functional, can be wear casual or elegant as needed and there are no unnecessary details. Of course there are many designs and few are great so you will need to research to find the right one. Always look at the quality of wool as the material will define the shape in a long term, I prefer thicker wool material. Depends on a design, peacoat should fit tight (but not too tight of course, you do not want to see stretched material around buttons) allowing you to put easily thick cardigan or sweater under. Sleeves length should reach wrist no longer. Collar shape is important as this will define if you could wear the coat with collar up or down. For example coat on my photos looks not really good when collar is down.
Go out there and research for a perfect peacoat and I'm always happy to help.