So I'm looking at expanding my pedal board in the near future, and I'm wondering what effects will work for both guitar and bass, and which ones would need to be bought specifically for bass. I'm building most of them myself from schematics, as I have little money for my rig, and I need to know what I can build that won't mess up my sound and what I need to buy. (schematics for bass pedals are very few) The main one I want to build is a compressor.

Thanks in advance
Nope, no sig here.
Well, any guitar pedal that emphasizes a specific frequency range: eq, envelope filter, wah, etc. is going to have no low end on bass. Distortions and fuzzes will not cross over well. Guitar pitch shifters or octave pedals may also run into tracking errors with particularly low notes.

As a general rule, most modulation effects are ok for both, but this really depends on the individual circuit you're using. Most flanger, phaser, tremelo, delay, etc will sound good on both. Chorus is the big exception, most bass choruses are a different beast than a guitar chorus and I wouldn't just mod a guitar one.

Look at some of the Mods on the Build Your Own Clone website, even if you don't use their kits, you can look at their bass mod instructions and some it might carry over if you find circuits that have similar sections.

edit* you might have this moved to the gear building forum, you can probably get a lot more help there.
Some of Boss' pedals are designed for both guitar and bass. I know the AW-2 Dynamic Wah can handle both.