For a quick reference I would look at these two links just for anyone that wants a refresher on EP prices, and quality, and the works.
EP Store
Follow up on EP Store
First, some of the kits you buy are specifically for adding roads and infrastructure, something that could not be done if you couldn't build on public spaces. I would not worry at all about whether you can add things like these, or watchtowers, as it would be pointless to add such things with the inability to place them where they actually make sense.
The addition of walls and roads of superior quality is not something I can find atm. Worst case scenario you could coordinate with your fellow players in the settlement in order to increase the number of uncommon quality walls/roads in the settlement, if the 1 per account is a firm rule.
Otherwise you would have to coordinate with the Count or Duke respectively. They do have access to kits that can help you add roads or walls as it is partially their responsibility to help you out.