[Fixed]-Django: How do I self-refer to a model but ignore common data fields?

1. I would like to avoid common fields when adding a subplot, for
   instance the address field, as it is the same as in the parent plot.
   What is the best way to do something like that?

You could make address as a property and change the address model field to _address. The property address returns the address of the parent if its own _address is empty:

class Plot(models.Model):
    name = models.Charfield()
    _address = models.Charfield(blank=True, null=True)
    _area = models.DecimalField(blank=True, null=True)
    parent_plot = models.ForeignKey('self', related_name='subplots') 

    @property
    def address(self):
        # here, if self.address exists, it has priority over the address of the parent_plot
        if not self._address and self.parent_plot:
            return self.parent_plot.address
        else:
            return self._address

2. Also, if a plot is composed of subplots, how could I set it up so
   that the area of the parent plot is the sum of all subareas.

Again, you can turn area into a property and make _area model field. Then you can do the following…

class Plot(models.Model):
    ...
    ...
    @property
    def area(self):
        # here, area as the sum of all subplots areas takes 
        # precedence over own _area if it exists or not. 
        # You might want to modify this depending on how you want
        if self.subplots.count():
            area_total = 0.0;
            # Aggregating sum over model property area it's not possible
            # so need to loop through all subplots to get the area values 
            # and add them together...
            for subplot in self.subplots.all():
                area_total += subplot.area
            return area_total
        else: 
            return self._area

maybe a good way would be to use inheritance. Create the main plots as parents and define every thing you want in those and whenever a child of a parent is created, specify what stuff the child inherits from the parent. Not sure if that helps