How to build a FiveM MLO: from idea to a custom interior on your server

A custom interior is what makes a FiveM server unmistakable: the police station that looks like your concept art, the club with the right floor plan, the workshop with room for six lifts. Technically such a room is called an MLO, and MLOs are considered the hardest kind of content to make for FiveM. This article explains why, what an MLO consists of and how you build one today without spending three months learning Blender.

MLO or YMAP: the difference

A YMAP places objects in the world: containers on a car park, barriers on a street, furniture in an existing house. The objects simply sit there; GTA does not know they belong together. An MLO (Multi-Level Object, called MILO in the game files) is a sealed interior with rooms, portals between the rooms and its own collision. Here GTA knows you are inside: it renders only the rooms you can see through portals, dampens weather and outside sounds, calculates light and shadow per room and lets peds and vehicles navigate correctly. That is why a well built MLO runs more smoothly than a YMAP with the same furniture, and why it is harder to build.

Which files an MLO is made of

• .ydr and .ydd: the models, meaning walls, floors, ceilings and every piece of furniture that is not from the GTA catalogue.
• .ytd: the textures. Every wall needs a material, every material a texture.
• .ybn: the collision. Without it you fall through the floor. Every triangle of the collision carries the number of the room it belongs to.
• .ytyp: the description of the interior. This is where the rooms live, the portals between them, which objects belong to which room and what properties a room has, for example whether it rains inside.
• .ymap: the placement in the world. Position, rotation and which ytyp entry appears there.
• _manifest.ymf: the table of contents that tells GTA which ymap contains which interiors. If the INTERIOR_DATA entry is missing, the room exists but is invisible.

The interplay of these six files is why MLOs are considered hard. A model is built in an hour. Making GTA understand it as an interior, with rooms, portals and a collision that matches every room, is the real work.

Route 1: Blender, Sollumz and CodeWalker

The classic route. In Blender you model the room, with the Sollumz add-on you create materials in GTA format, build the collision as its own object and define rooms and portals by hand. The export yields ydr, ybn and ytyp. In CodeWalker you place the interior in the world and generate the ymap and the manifest. For people who master Blender it is the most flexible route: there is no shape that cannot be done.

The price is time. Expect two to four weeks of learning for a first usable room, and errors that produce no message: a portal half a metre off the doorway makes walls flicker as you walk through. A collision without a room number lets you fall through the floor, but only in one room. A material with the wrong shader renders black. None of this shows up in the console.

Route 2: 5MC Forge

Because we made exactly those mistakes for years ourselves, we built Forge. It thinks in floor plans instead of files: you draw rooms on a grid, place walls, doors and windows, pick floors, walls and ceilings from a texture library of GTA surfaces or your own images, put in furniture from the GTA catalogue or custom models and set lights. Rooms, portals, collision with room numbers, ytyp, ymap and manifest are generated at build time because Forge knows the floor plan. A test server button starts a FiveM server with the room so you can walk it right away.

What Forge cannot do: free shapes. A room is built from walls on a grid; curves and sloped ceilings are models you import. For police stations, hospitals, workshops, clubs and shops that is no obstacle; for a cathedral it is. In return, a room that costs a week in Blender takes an afternoon in Forge, and it works on the first build.

The rules GTA imposes

1. At most 32 rooms per interior. The collision only knows room numbers 0 to 31; anything above is not drawn. We learned that from a police station whose upstairs office simply did not exist. Large buildings are split into several interiors.
2. Portals must be honest. A portal is the surface through which you see from one room into the next. If it does not sit exactly in the doorway, the wall behind it flickers as soon as you walk through.
3. Objects must not be newer than the server. A single piece of furniture from a DLC the server build does not know makes the whole interior vanish, with no error. Check which build the server runs before building.
4. Scaling objects does not work in the world. A prop that is half size in the preview stands at original size in game. If you need a smaller pane of glass, use a different model or a custom panel.
5. Every surface costs streaming memory. A room with 4K textures on every wall is beautiful and makes a busy server stutter. 1K for walls, 2K only for surfaces you see up close.

Installing it on the server

An MLO is a resource like a vehicle: a folder with fxmanifest.lua and a stream folder holding all the files. The manifest needs one line telling GTA that map data is coming.

fx_version 'cerulean'
game 'gta5'

this_is_a_map 'yes'

Then ensure it in server.cfg, restart the server, teleport to the position. If the room replaces an existing GTA building, say the Mission Row police station, the original has to be hidden. Forge has a list of the models that should disappear at that spot for this; without such a list both buildings stand inside each other.

The mistakes everyone makes once

• The room is invisible but you bump into walls: the manifest does not know the interior, or an object comes from a DLC that is too new.
• You fall through the floor in one room but not the others: that room's collision carries the wrong room number.
• Walls flicker as you walk through: a portal is not in the doorway or missing altogether.
• Everything is dark or flatly lit: the room lacks light sources, or it has no timecycle modifier allowing interior light.
• It rains inside: the room is missing the flag that keeps weather out.
• Doors do not open: doors are objects with their own logic and need a door script on the server that knows them.

MLOs from 5MCustoms

We build interiors as commissioned work: to your floor plan, with your logo on the wall, at a fixed price from the cost estimate. Every room is tested on a server with the customer's build before delivery. If you want to build yourself and you are a mapper, you can reach out through the mapper programme and work with Forge.

Justin Beil

Founder of 5MCustoms

Justin has been building FiveM servers, vehicles and scripts for years. 5MCustoms is the framework that grew out of it. What you read here comes from practice, not from a product sheet.

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