I searched, but I couldnt find it: when I place a garden, should I limit my placement to a place I can reach by road?
The paragraph about the garden placement doesnt have any map restriction
Place new houses and gardens
If your structure contains a new business developer, you may place a new house on the map, or add a garden to an existing house.
Placing a new house: A new house may be placed in any empty area on the map, provided the house is connected to at least one road. There are no range limitations. You may freely choose which house to place.
Placing a new garden: A new garden must be placed so that it connects to a house printed on a map tile. It has to connect over 2 squares, so that the final house and garden forms a 2x3 rectangle. Gardens may only be placed on empty squares. Each house may only get one garden. Turn the garden so that it is obvious which house it belongs to. Houses that have been placed on the board by a new business developer automatically come with a garden, and may not get another one.