Crafting/improving Magic items

[Dericof Diname]

Hello guys, I'm having some problems and doubts about improving magic items.

My character have a Maul +1 and I want to improve it by adding thunder damage dices on it.
I was thinking a +1d8 thunder and it but making it a Attunement item plus 2000 gp extra cost.
However, one of my fellow playmates keeps telling me that the rules says that the lowest spell with thunder damage is 2d6 and is an area of effect.
I'm kind of lost here and my DM is is still learning about the system.

How does improving a magic item works on d&d 5e?, Are there some rules about it?, How does this works?

Also if you can provide with some references, like book and page, so that I can support my argument with DM.

Thank you.

[JackPhoenix]

It doesn't. Spellcasters can create magic items, if they have appropriate formula. Unless you're creating a wand or a scroll, it doesn't matter what spells do. That's it, as far as rules are concerned. 5e wants to keep magic items rare and thus avoids having them craftable by players. It's Dungeons & Dragons, not Workshops & Craftsmen.

Now, the DM can allow other things, but you must ask him, there are no rules for that. All relevant rules (what few there are) are on pages 128-129 in DMG.

[Foxhound438]

If you want to do more of a homebrewy-do-whats-fun kind of thing, look at "Elemental Weapon". Switch out the "+1 atk, +1 dmg" to "+1 atk, +1d4 thunder damage". There's no real way to handle this by RAW, but in previous threads it's been agreed that that would still constitute an uncommon rarity, by power level, so another 500 GP might be appropriate as the cost for that effect.

[Renvir]

DMG 284-285 might help you out a bit. I'd also suggest looking at DMG 170-171 regarding Flame Tongue and Frost Brand, two weapons that also have bonus elemental damage. Combined with DMG 128-129 you should have enough information to make a guess as to how the weapon could be created and what its effects would be.

My 2 cents regarding your thunder damage maul. If your DM does allow you to craft this weapon then your maul is already an uncommon weapon due to the +1 attack and damage. Adding elemental damage on top should probably move it up to a rare level. Creating a rare item costs 5000gp but since the +1 is already there I'd cut it to 2500gp. As for the damage amount, I'd either go with Foxhound438's suggestion of +1d4 thunder or 1-2 times a day you cast thunderwave as a bonus action after a successful melee attack. Besides the gp cost I'd also say you'd have to cast Elemental Weapon or Thunderwave once per day throughout the crafting process as well.

If you want to stay away from crafting, I'd suggest asking around the bars, guild halls, libraries, forges, etc. for information on someone who'd be willing to craft/enchant the item for you in return for gold, a service, or both. This gets you out there exploring, your party members won't hate you for wasting 300 days of in game time, and keeps you away from micromanaging item crafting.

[Dericof Diname]

Thanks, for the tips.
Yeah, my plan is to go to the markets and special stores in order to look to someone who can provide the enchantment for my weapon.