Test it a whole bunch and make changes and keep updating the optimization while doing that.
Slowly fill in the map and improve and refine it and keep thinking about how to change the structure to improve the optimization and gameplay.
You can add broad layers of detail as you go and as you make changes, it will add up. I'd suggest not fully detailing room by room as some do but rather go in broad sweeps and layers of art accross the map.
When artpass:
0. Dev textures, ground textures, sky, basic env light, 1 cubemap, water control, large props layout and optimization essentially done
1. Base textures
2. Detail stairs and doors
3. Adjust optimization and structure
4. Func_details and rough out of bounds areas.
5. More textures
6. Overlays and more models
7. Refine lighting
8. Fog?
9. Adjust structure for gameplay and optimization
10. MORE ART!
11. fade distances on props, mindxlevel on non colliding props?, func lod?
12. adjust lightmap scales
13. refine art so each area has clear gestalt
14. optimize
15. more cubemaps and finalize art and lighting
(optional)16. find out everytihing is wrong.