Hello there!
I’m not very experienced with subdivision surface modeling, and have come across this problem, I want all the edges to be smooth and join up with the big mesh, but when I apply the modifier it creates a mess.
Thanks!
You have ngons- faces with more than 4 edges. They don’t subdivide well and are creating the artifacts you’re seeing
Hello!
Thanks for your help! I can see that now. How would i fix this problem? Add more cuts to my mesh?
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It depends on if you’re planning on deforming and animating your mesh. Assuming you’re not, go to the Select menu and choose Faces by Sides. Type in 4 and choose the Greater Than option. With the n-gons selected, press… I think… Ctrl T to triangulate. That should help quite a bit 
Yes! I tried on the whole mesh and it turned into a potato 
but I think with some precise adjustment that could work.
Thanks for your help!
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If that method doesn’t work, you can select all the n-gons like previously outlined, press I to inset, move your mouse slightly to the left to scale the inset faces down, press Ctrl I, and press H. You’ll be left with only the inset faces visible. Going each face, one at a time, select all the vertices of the Face. Press M, choose at Center. This way, you’ll merge each inset face into one vertex. This should preserve details better
Also for future reference, I’d recommend gaining somewhat of an insight leveraging this methodology:
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If it’s a hard surface model that does not need to be deformed/animated then it’s often best to not rely on subdivision.
Check out the subdiv and hard surface videos from Arrimus 3D . If you are comfortable enough with 3d in general you should be able to learn a lot from any of his videos, even the ones not using Blender.
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Instead of using subdivisions, bevel the sharp edges.
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Fantastic! Thanks for everyone’s feedback!
Appreciate it 
Note: I’m not aware of your experience, so I’ll explain things generally, which may cover what you already know.
Referring to what was mentioned regarding Quad (4-Vertex) Topology:
This [Image] is a quick, rough example of what would be considered Quad Topology for ~half of this current model. (Red, X marks mean I recommend removing these points, at first).
While N-Gon’s (5+ Vertices to a Face) aren’t generally always bad, they should be avoided for most subdivided meshes, specifically in areas which may require deformation or areas which fall on curves/corners. It’s best to keep things simple and in Quads to get a clean block-out model before considering adding any SubD/Bevel/similar modifiers to your mesh. A good foundation mesh is key.
Generally a SubD/Hard-Surface workflow goes as follows:
Create a High-Poly mesh with clean subdivisions and smooth, light-catching, beveled edges (preferably done in a Modifier/Non-Destructive-based workflow). Then from that High-Poly mesh, you either “extract” a Low-Poly version (remove any SubD/Bevel-related modifiers) or create an entirely new Low-Poly mesh which matches the exact profile and origin(s) of the High-Poly.After both the HP & LP meshes are complete, you only UV Unwrap the LP and export both as “.obj”. Then in a texturing software like Substance Painter, you Bake the HP detail onto the LP mesh, making the model’s geometry more well-optimized, while still keeping the good HP Normal/Mesh details (such as the bevels).
This same general workflow is used for Organic, Sculpted Meshes, such as Characters and complex Props (clothing, bags, rocks, etc.).
However, to elaborate on what was also previously mentioned, regarding Bevels: I strongly recommend using the Bevel Modifier in your Modifier stack, on top of the SubD for something like this (set to a relatively small number). Set just the Geometry > Milter Outer to “Arc” for generally better corner/edge results with SubD, as this method will create Quad Bevels for those areas instead of Tris (3 Vertices to a Face).
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In addition to my previous reply:
A reason as to why you’re seeing very wide depth/curves on your mesh after applying SubD, is due to your lack of “Edge Loops”. These will act differently than Bevels, but they share a similar purpose for SubD/Hard-Surface workflows.
The Images below illustrate an example of where the same SubD Modifier is applied to two identical Cubes. One cube has no Edge Loops, and the other has an equal amount surrounding all of it’s edges.
Edge Loops (in this case) will help create/reduce what is essentially a boundary which will in-turn determine the depth your SubD modifier will be allowed to deform your mesh. (There is more to Edge Loops, including how it pertains to Normals, but I won’t go into it here).
Keep these concepts in mind when developing the rest of your mesh, as they will all play a vital part in your end-result, especially when it comes time to UV, Bake, and Texture your mesh.
Good luck, and I hope all of this helps!
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I also recommend this chapter from the Lightwave manual. Its author has become a member of these forums and is now a Blender user. https://docs.lightwave3d.com/lw2020/appendices/user-created-tutorials/fundamentals-of-subpatch-modeling Ignore the lightwave specific things and keep in mind that subdivision surfaces work the same in all programs.
This seems a good place to expand on the question. I agree with the general consensus about the geometry to avoid and to subdivide and keep the shape needs extra geometry.
Let’s say OP was going to use this object in a larger project, would it be advised to keep a similar poly density across objects?
If planning on using subsurf, do you generally turn it on early, or once the overall ‘shape’ is complete?
I have often seen advice from seasoned experts to leave it off until your shape is nearly complete. I think as long as you follow the guidance of “use enough geometry in the base mesh and don’t overly rely on the side effects of subdivision as a modeling tool” then you can turn it on whenever.
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