I was playing with a DM that says the rules state that you cannot have a 1st-level character with an ability score above 18. I didn't want to argue so I made it work. I cannot find that rule anywhere.

We are using the "roll 4d6 for each ability score, drop the lowest" method of generating ability scores.

Is there a rule that states a 1st-level character cannot have an ability score above 18?

There is no such official rule.

Chapter 1 of the PHB/basic rules, titled "Step-by-Step Characters", lists how to determine ability scores in step 3 (italics added for emphasis):

You generate your character’s six ability scores randomly. Roll four 6-sided dice and record the total of the highest three dice on a piece of scratch paper. Do this five more times, so that you have six numbers. If you want to save time or don’t like the idea of randomly determining ability scores, you can use the following scores instead: 15, 14, 13, 12, 10, 8.

Now take your six numbers and write each number beside one of your character’s six abilities to assign scores to Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma. Afterward, make any changes to your ability scores as a result of your race choice.

Using the "4d6, drop the lowest" method of ability score generation, the only limit on what you can roll is what the dice literally limit you to: a maximum of 18 (unlikely but possible, if you roll at least three 6s), and a minimum of 3 (very unlikely - only possible if you roll four 1s).

However, these numbers are the base stats, before racial modifiers are applied. So if you roll at least one 18, and you pick a race that gives +1 or +2 to an ability score, you can start with a score of 20 in that ability.

(The point-buy variant for generating ability scores does have stricter minimums and maximums on what ability scores you can have, but you're not using that method so I won't go into it in detail.)

The DM can add house-rules - but they should make it clear when they're doing so.

D&D empowers the DM to run their game as they see fit, and add house-rules if they so desire. However, in your case, it seems like the DM seems to think there's an official rule preventing characters from starting with an ability score of 19 or 20 after adding racial mods. There is no such rule.

I would recommend asking your DM to go through the character creation rules with you, and politely asking them to point out the rule that they think states you can't start with an ability score of 18 or higher. Otherwise, if it's a house-rule, you should ask your DM to make that clear. Then you can decide whether you're willing to accept that house-rule or not.

This is either a rules misinterpretation or an intentional house rule.

V2Blast's answer is correct, but I'd like to add some context for why your DM might have drawn the conclusion that ability scores can't start above 18.

Page 7 of the Player's Handbook notes:

The abilities are Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma, and they typically range from 3 to 18 for most adventurers. (Monsters might have scores as low as 1 or as high as 30.)

1. The DM may have misinterpreted this rule to read that 3-18 is a limit, rather than a tendency.


2. A miscommunication may simply have occurred between the DM and player. For example, the DM may have misunderstood, when you mentioned a stat above 18, that you were referring to a natural roll above 18, which you can't have (at least not on 4d6 drop lowest).


3. The DM may have intentionally house-ruled to prevent overpowered characters. In my D&D 3.5 games players often combined an 18 with a +2 to start with a 20^†, and such a character goes around overpowered compared to their peers and is more effective than a character of their level is expected to be. It's not entirely unreasonable for a DM to want to nerf this with a house rule.

Note: In situations like these I've previously seen suggestions to confront the DM and ask something like, "Just to clarify, you are aware that you're using a house rule, right?" I don't like this solution personally, as it can come off as passive-aggressive. DMs often make rulings based on what feels right or what worked well for them in previous games, and don't necessarily want to have their authority questioned.

^† And my players rolled a suspiciously high number of characters with at least one 18 before racial bonus. D&D 3.5 in particular really rewarded players for starting with the highest possible ability scores, so allowing ability scores higher than 18 created a strong motive to cheat.

V2Blast's answer is very good, but he skips over the other method of starting attributes which is another official one:

Point buy. With this method there is a limit of 15 without racial boni on the starting attributes. So yes, when using the point buy method a character can not have more then 17 in an attribute at the beginning of the game.

In your specific case, this doesn't apply as there is no limit when using the rolling method, but you can see where your DM might be confused, and what rule he might be thinking about.