@Iordan_Iordanov, if your goal is to map a square grid in texture over an in-game mesh, you can accomplish this with a custom node material. Your geometry will have your normal texture set using PBR, blinn-phong, or custom lighting models, but you also add in another layer into the shader to create your grid. This can be a simple matter of adding grid lines in the shader math, which will give you the control of changing the size of the grid on the fly if you want (or changing it based on the underlying geometry).
Then you can simply do a raycast and get the UV coordinate of wherever the raycast hits the mesh to know what cell should be highlighted in the shader. You could even pass part of one of your textures (an unused channel like alpha on a normal texture or metallic/roughness texture) to pass a map of eligible spaces for placement.
For example, if you have a piece of geometry that has a bridge on and water beneath the bridge, maybe you only want to be able to place units on the bridge. You can use an alpha texture to differentiate between valid and invalid spaces where your bridge is white in the alpha channel and all other parts of the texture are black. Then you can simply use that key when rendering your grid to show where your grid cells should or should not be placed. That way you can customize the grid per piece of geometry rather than needing to build your geometry to the grid.
If you approach it this way, the cost is negligible in that you have some more work to do in the shader, but no additional meshes to load. Here are a couple of examples of using a shader to create a grid on a mesh. Neither of these are an exact solve for what you are doing, but they both can be used as reference for your custom solution.
Hex grid post and playground
How to light hexagon (or an specific zone) in height map floor? - Questions - Babylon.js (babylonjs.com)
Node Material Hex Grid Highlighting | Babylon.js Playground (babylonjs.com)
Square grid on sphere thread and playground:
Creating quad sphere - Questions - Babylon.js (babylonjs.com)
Rubik’s Sphere | Babylon.js Playground (babylonjs.com)