Is it illegal to build a fence on the large balcony that comes with the purchase of a house?

Regardless of whether it is illegal or not, there is a fatal danger in doing so, because the structural reinforcement at different positions of the building is matched according to different forces. The balcony is a completely different place from the interior structure, and if you want to remodel it for indoor use, you have to figure out if the force is enough. Also, it must be illegal for you to build a fence on the roof balcony.

Of course, as long as it is not in the original planning of the building structure, it is an illegal building (it is illegal or not illegal, in violation of the planning law), even if it is some steel frame structures and the like.

Not counting the floor area. If the floor area is to be calculated (also only under the balcony), the following conditions must be met at the same time: the two sides of the fence wall coincide with the two sides of the balcony on the second floor, the two ends of the fence are connected to the wall under the balcony, and the height of the fence wall is to the bottom of the balcony (the meaning of this article is:

The fence can actually be used as an envelope for the lower space of the balcony); Under the balcony there is a door from the interior to the outside; There is no lawn or greenery directly below the balcony, and it is a hard floor. When the above three items are reached, the floor area below the balcony can be calculated according to the door bucket or eaves corridor according to the first article in GB T50353 2005. If there is no balcony or structure above, no matter how much you can't calculate the construction area.

For the calculation rules of construction area, please see: GB T50353 2005 Construction Area Calculation Code.