How much disk-space is needed to store a NULL value using postgresql DB?

Laramie is right about the bitmap and he links to the right place in the manual. Yet, this is almost, but not quite correct:

So for any given row with one or more nulls, the size added to it would be that of the bitmap(N bits for an N-column table, rounded up).

One has to factor in data alignment. The HeapTupleHeader (per row) is 23 bytes long, actual column data always starts at a multiple of MAXALIGN (typically 8 bytes). That leaves one byte of padding that can be utilized by the null bitmap. In effect NULL storage is absolutely free for tables up to 8 columns.

After that, another MAXALIGN (typically 8) bytes are allocated for the next MAXALIGN * 8(typically 64) columns. Etc. Always for the total number of user columns (all or nothing). But only if there is at least one actual NULL value in the row.

I ran extensive tests to verify all of that. More details:

• Does not using NULL in PostgreSQL still use a NULL bitmap in the header?

Null columns are not stored. The row has a bitmap at the start and one bit per column that indicates which ones are null or non-null. The bitmap could be omitted if all columns are non-null in a row. So for any given row with one or more nulls, the size added to it would be that of the bitmap(N bits for an N-column table, rounded up).

More in depth discussion from the docs here

It should need 1 byte (0x00) however it's the structure of the table that makes up most of the space, adding this one value might change something (Like adding a row) which needs more space than the sum of the data in it.

Edit: Laramie seems to know more about null than me :)