Wire Cat5 to Adjacent House

You'll need to sleeve the cable and then bury it, ethernet is good up until about 90m to 100m give or take so the distance isn't an issue from a technical perspective.

Any other recommendations for burying the conduit? From my research it should be between 18-24 inches. Some of my colleagues have recommended having an electrician come out and setup the conduit.

I would like to do it myself if possible, but as I've not done anything like this before I'm not sure what equipment I'll need or what to expect. Are there any guides online you have come across that could shed some insight?

Does the property between the two houses belong to your client? (I will assume it does)
As Alex states, you'll need to run the cable/bury it.  I understand you can purchase cat5 designed for external use, though I haven't seen what the difference is between the two.

to be honest, if it's at your home you can simply drill through the wall, just make sure you don't go through anything.

https://www.homedepot.com/p/3-in-x-10-ft-PVC-Schedule-40-Conduit-67512/100123139

That'll do it, just thread the cable through.

Regards

Alex

Building and electrical codes vary widely from state to state and country to country but this shouldn't be terribly difficult, but you need to know the requirements in your area. Generally, low voltage cabling such as Cat5 Ethernet doesn't need to meet the same codes as burying electrical cable (AC). Six inches should be fine and can be buried without conduit if you use something like Ubiquiti Toughcable which is shielded and coated for outdoor use. Conduit is always nice though as you can then run more networking cable through it later. With conduit you then don't need the more expensive outdoor rated cable as long as it's not exposed to the elements at any point. Direct UV rays from the sun will make the plastic coating on indoor rated riser cable brittle and crack within a few years.

Still, if you're going to dig, call the "dig free" service in your state to check for any underground service lines first. And, if the neighbor is paying for it, save yourself the back-breaking work and have him hire a contractor with trenching machine which will dig that sucker in a few minutes! Good luck!

I have run Ethernet in a building for nearly 100 meters (not buried) and it works fine.

I agree with the above suggestions to bury it in a conduit below any frost level.  Run the conduit so that it is fixed in place and then fish the Ethernet through.

This makes the Ethernet replaceable if need be.

Speaking of fishing the cables and the rope, you will need either (a) an electrician's fish tape, or (b) 10 foot sections of drain tile.  (b) is the way I'd tackle it.

Drain tile is corrugated so it is difficult to run a fish tape through once the conduit is in place, and it is unlikely that you would be able to locate a cable pulling robot for rental.  Much easier to run the cables and rope through 10 feet tile segments one at a time, above ground.

Depending on the area (trees and other obstetrical) you might consider running a fiber line above ground between the two buildings. Just be advised, should you do so, you'll probably want to secure it to a steel cable between the buildings rather than JUST draping the cable alone - better to support the weight of the cable and survive high winds, frozen water, etc.  (That's what we did with one client to cross a street).

Do check with the local officials as there may be ordinances that prohibit such things.

You do not need to bury it below the frost line; it doesn't matter if it freezes.  18" is a good depth but you can go less if code allows.  Since it's only 40' I'd suggest just using a Mattock to dig the trench.  I'd suggest keeping it inside Sch 40 conduit to protect it but it's not 100% necessary.  Regardless of whether you put it in conduit you should use Gel Filled cable to protect against water.  ALL conduit buried in the ground eventually fills up with water, period.  Don't let anyone tell you otherwise.

18 inches (even 24 inches) should be fine.  Too close to the surface and conduit can crack / break with ground heaves.

After reading everybody's advice I feel very confident to do the job, so thank you! The only issue I'm running into now is the city.

I got in contact with them, and they prohibit running a cable between two property lines.

I'm going to give them a call today to get further information, but it looks like we may be stuck using a point to point wireless system, or getting a new internet line setup.

Any thoughts on how to proceed?

A conduit between houses is not a big deal. Perhaps ask the city if they can provide a more acceptable opinion.

If not, the owner will have to accept point to point wireless.  Cheaper than a new internet line.

Thank you for the help everyone, looks like we'll end up using point to point wireless. But at least if an outdoor install on the same property comes up in the future I'll know how to handle it.