Im curious when doing modular building if I have a room that is 250x250. Do I make the walls 230x230 to account for a 10 width on each wall? If it’s 250x250 you get overlap. Have to fill in a corner piece?
Hey @Xokks!
The trick here? Overlapping is fine! 
What the player doesn’t know won’t hurt them. You’ll notice a ton of games will have a corner piece such as a corner brick set, or a column to hide the seam! Though I would suggest- keep it to multiples or divisors of 100 to start with. Such as 300 unit width walls, 200s, 100s, and 50s. 
Hope that helps!