Translation:Go to his house inland of the road.
I think inland and seaward are perfectly good American English directional terms - and I live in a very land-locked part of the country. "Upland" seemed a little odd... but then, aren't all the Hawaiian Islands basically the tips of volcanoes? If so, I can see how the terms would be interchangeable.
What does "inland of the road" mean? Is it standard American English? We would not say it in British English.
No, it's not standard American English. I think it's Hawaiian English. I think it makes sense for people who are used to living on islands that are small enough and where most things are close enough to the coast that inland and seaward are immediately obvious and relevant directions to everyone at basically all times.
This is the impression I was given too (I'm an American not from Hawai'i as well).
I grew up in Hawaii. The words mauka (ma uka) and makai (ma kai) are used locally to indicate direction -- mauka is towards the mountains, makai is towards the sea. The terms are only used when it is obvious which direction is which -- this is often the case as the mountains here are visible from most / all locations.
Aloha! I think this would be "the internal side" of the road, where a side is away from the coast, the sea, and the other side is the seaward, next to the coast, in the direction of the sea.
Some related words:
https://en.wiktionary.org/wiki/seaward (adjective, adverb)
https://en.wiktionary.org/wiki/headland (noun, area)
https://en.wiktionary.org/wiki/hinterland (noun, area, remote place)
I hope it helps. ;)
There are or were roads that circled the islands, as such, you can tell someone to look for a house on either the seaward or inland side of that road. If they live closer to the pali, within the hills or mountains, you can refer to the uplands.