It always bugs me when people say that 5E isn't dangerous enough. I stated it often on this forum: "I have seen more TPK in 5E than in any other edition" And I don't have 4 or 5 groups playing every week as I had 35 years ago...
Do I have house rules? Yep, almost the entirety of them on the players' side. Only my BBEG houserule is strictly on the monsters' side (and I've had one for 2nd, 3.XE and 4E) and even then, it only applies to a party that is bigger than 4 characters...
What I do apply though, are the followings (all can be found in the DMG, XGtE or other source books.)
1) No healing on rest. You need to spend magic or HD to heal.
2) A short rest is 10 minutes or an hour (players will know in advance and will have my reasoning behind it) a long rest is 8 hours. Only two short rests are allowed between long rests.
3) I strictly adhere to the 6-8 encounters per day. If any attempt to make a long rest or more than two short rests are made, the festival of random encounters starts.
4) Random encounters never gives any treasure or experience. I have done this since 2nd edition. I want the characters to do adventures, not farm experience on random encounters and load out on random treasure.
5) The whack a mole is severely punished. If a character falls, use spare the dying. IF you are up against intelligent foes, this will work out once. After that, every foes will attack the downed PC so that he dies then and there. This encourages players to actually retreat from hand to hand to get healed up. Use the dodge action and other shenanigans to avoid going down. Even a monster of low intelligence might decide to get the downed PC and run off with the body to get a nice snack away from harm.
6) Monsters react to the PCs actions. Intelligent foes will pursue and form seek and destroy squads. Build more traps, reinforce rooms and seal off what can be sealed if fleeing is not possible. Beast like foes will flee if given the chance.
7) As the players are getting stronger and get some reknown, key evil NPC will start to hear from them (and the reverse is true, the players will learn about stronger vilains from reputations). Players that slay everything in sight will not have a break if captured. If they can be reasoned with, they might/will get ransomed for their lives.
8) Downtime activity is about one week to 10 years depending on the circumstances. So creating scrolls and potions is quite feasible. There are no magic shop, but it is always possible to get your hand on a magical item you want by petitioning the mages' guild to make one for you. Expect a stiff price and a few quests for them, depending on the item. Common items are no big deal (or ammunition) but permanent items and weapons are usually at a premium price. There are a lot of gold sinkers in my campaigns.
These are not in any books. These are personal preferences that I have developed over the years.
1) Religion matters a lot. And I mean really a lot. You will not be raised from the dead by a priest if you are not a follower of the god. The god will simply not allow it. An ally might get raised, but the cost will be high. You're party cleric will not be allow to raise you back from the dead if you are not a follower of his/her deity. A bard or any non religious caster attempting to raise a character will enter in communication with the deity of the player. It is entirely possible that the deity might not want the character to be resurrected if he was never giving tithe to the church. Non religious characters, atheists, agnostics, will be judge by the god of death (Kelemvor in FR, Wee Jaas or Nerull or the Raven Queen in Greyhawk) and depending on the circumstances, sacrifices might be in order in addition to the cost of the spell itself. For these reasons, most players will try to avoid death.
2) Social standing matters a lot. Most countries in D&D are not democracy and do not work like a democracy. Adventurers, though useful, are not nobles. They can not just enter anywhere and do as they please. A noble will always take precedence and ignoring rank and being disrespectful will ensure yourself to be severely punished. If the adventurer is too powerful to be arrested by normal means, assassins will be hired to remedy to the perceived slight the noble suffered if evil, or they could become personae non gratae if good. Being ostracised might even be worse than death as no one will want to slight the noble. Shops will close or will not sell to the players.
3) Thieves guilds matters too. Yes, protection is a racket. But better pay up a few gold pieces than being targeted for theft. Even Nobles know that a Thieve's Guild is a necessary evil. As the guild will pursue and arrest rogues thieves when the start operating on their territory. If the character is too high level, offers will be made. Compromised will be offered. And if refused, assassins will be hired...
4) Ransom is a good thing (well sort of...). If a villain is captured, ransoming him/her is always a good thing. It makes money to the players and it makes for a good recurring villain. And it might give clues as to the villains' lord... The players can expect the same treatment if they too, act this way. Even low level shmooks can be ransomed. So having a place to hoard you gold in case you are ransomed is a good thing. Thus the protection racket from the Thieve's Guild... Thus the tithes given to church, Thus the good relations with nobles and yaddi yadda...