How to setup the structure in Craft for a Property Development site I'm building
I think I follow where you are going. It sounds like you have a few levels so you will indeed be working with a structure (instead of a channel or single).

I'm not sure what "All Developments" means - is that "show me every single entry in this section"?

There are most likely a handful of ways you could go about doing this, here is one way that I would try.

1.) I would make your "County" a category. Call the group "Counties" and then create each county you need (ex: Berkshire). By creating your counties as categories, you can assign (relate) anything back to it. This will allow you to do things like: Get all properties from Berkshire Country. It will be up to you to decide where this relationship lives though. I would put it on the actual property though, that way you have access to it at any level.

2.) Create a few entry types:
        1) Development
        2) Property
        3) Plot
     By doing this, we can have a clean authoring experience by having each entry type have it's own fields. Meaning your Properties entry won't have inputs for the images like your Plots entry would. It just keeps things a lot cleaner for the author.

3.) Build out the structure for each Development. This is what that might look like. Let's say you have a section (of type structure) called "Developments"

 - Entry: (Type Development)
     |- Entry: (Type Property) &lt;-- This is where I would assign County
         |- Entry: (Type Plot)

 - Entry: (Type Development)
     |- Entry: (Type Property)
         |- Entry: (Type Plot)


By setting things up this way, now we can easily do things like, "Get all plots from foo property" or, "get all properties from bar development". 

Or, another completely different way would be create categories for each County, Development and Property. You would then have a single section called "Plots" (of type channel). Each entry would then have category inputs to relate the plot back to the correct County and property.

The problem with this approach is that it isn't quite as straight forward to create higher-level things like Properties. I've found that having people go back and forth from categories and entires can be confusing.

Hope this helps get you going!