How do I start building on a land?

[Charlton Ho]

Hello experts,  I need some help here since I couldnt really find good answers online. I made a purchase of a lot with a duplex in OR which according to the city I can have 4 units in total. There's an empty land next to the duplex and I already hired a landsurveyer to draw the lot boundary with marking on the ground. I'm hoping to build another duplex on that land. and I'd like to put a nice trailer as an airbnb place while I gather the money or do the pre-work of building the duplex.  Here are the parts I need help...

- Who are the people I need to contact to get things going?  I already have the site-map but I believe I need to get the land prepared and planned for my goal.  I dont know whom I need to work with to get there...

- The empty land doesnt really have direct road access.  there's a tree in the middle.  Who should I find in that case to build an access road to the land?  without that, I probably cannot even get the trailer on the land

Thanks for reading and the help in advance.