They aren't really within the same universe. Originally 40k was Warhammer Fantasy in space, but it evolved away from it.
There are some things that they have in common. The Greenskin races are in both, humanity (obviously), the four Chaos gods are all present in both, and the Old Ones are also mentioned in the background for the Lizardmen and the Necrons.
There are some odd theories I got that might mean that they are connected. Sigmar is a total beast in Warhammer. Perhaps he might be a Primarch? He seems to be similar to the other Primarchs before they were found by Imperial forces.
If I may explain an odd theory I have, the Great Maw from the Ogre Kingdoms has caught my interest. A giant green meteor falls out of the sky and devours its way through the Warhammer world. The interesting thing I find about it is that it was said that before the Great Maw meteor hit, it was growing green and had a massive mouth on it. After it hit, the ogres began to feel a hunger like never before. They are now driven to consume anything and everything for their ever-hungry god. Switch to the 40k universe. The Necrons are lead by the C'tan, star gods of immense power that hunger for the souls of anything living. Right now it is said that only 4 remain. Two are described in the Necron Codex: The Nightbringer and The Deceiver. The third is heavily hinted at being the Void Dragon and is worshiped by the Mechanicum on Mars. This leaves one more unknown. My theory: the Great Maw is this final C'tan. It fell from the sky one night obliterating a huge area of land. It has caused an unnatural hunger to prevail throughout the Orges and its always hungry. It's just my theory though.
The one thing that kinda separates the two universes is that the 40k-verse is our timeline 39,000 years in the future. So unless the Warhammer world is a yet undiscovered planet, it isn't part of the 40k-verse.
Edit: Although they might be connected by the warp. It wouldn't surprise me to find out that the Chaos Gods are manipulating two (or maybe more!) universes.
There are some things that they have in common. The Greenskin races are in both, humanity (obviously), the four Chaos gods are all present in both, and the Old Ones are also mentioned in the background for the Lizardmen and the Necrons.
There are some odd theories I got that might mean that they are connected. Sigmar is a total beast in Warhammer. Perhaps he might be a Primarch? He seems to be similar to the other Primarchs before they were found by Imperial forces.
If I may explain an odd theory I have, the Great Maw from the Ogre Kingdoms has caught my interest. A giant green meteor falls out of the sky and devours its way through the Warhammer world. The interesting thing I find about it is that it was said that before the Great Maw meteor hit, it was growing green and had a massive mouth on it. After it hit, the ogres began to feel a hunger like never before. They are now driven to consume anything and everything for their ever-hungry god. Switch to the 40k universe. The Necrons are lead by the C'tan, star gods of immense power that hunger for the souls of anything living. Right now it is said that only 4 remain. Two are described in the Necron Codex: The Nightbringer and The Deceiver. The third is heavily hinted at being the Void Dragon and is worshiped by the Mechanicum on Mars. This leaves one more unknown. My theory: the Great Maw is this final C'tan. It fell from the sky one night obliterating a huge area of land. It has caused an unnatural hunger to prevail throughout the Orges and its always hungry. It's just my theory though.
The one thing that kinda separates the two universes is that the 40k-verse is our timeline 39,000 years in the future. So unless the Warhammer world is a yet undiscovered planet, it isn't part of the 40k-verse.
Edit: Although they might be connected by the warp. It wouldn't surprise me to find out that the Chaos Gods are manipulating two (or maybe more!) universes.