A few weeks ago, Jinja Shinpō had a full-page special about a jinja that has recently built a new jinja office. This sort of article is fairly common, because it is an important event for the jinja in question, and positive news to share with the Shinto community as a whole. The jinja in question is in Tokyo, which probably explains how they can afford to do it, and the new building does look nice, and very useful for matsuri preparation: it has a kitchen for preparing offerings, for example, as well as places for people to wait.
One point was particularly interesting, however. The office is built next to the prayer hall and main sanctuary to the jinja, and across the lobby from the prayer hall is a Japanese-style room, which contains a “saidan” in the wall furthest from the sanctuary. A saidan is, literally, a “matsuri platform”, and looking at the photograph this one is a niche in the wall with bamboo screens, where offerings and such can be placed. Strictly speaking, I think a home kamidana counts as a kind of saidan, and there are also temporary ones that are set up in order to perform matsuri away from a jinja.
So, why do they need a saidan so close to the prayer hall?
It is for performing memorial matsuri for the dead, and for the matsuri performed at fixed intervals after someone’s death. (In Shinto, the traditional intervals are fifty days, one year, and ten years.) These matsuri cannot be performed in the prayer hall, in front of the main sanctuary.
Why not? Death is the largest source of kegarë, ritual pollution, in Shinto, which is why funerals are never held at a jinja; instead, the priest goes to a funeral hall and performs the ceremony there. Bringing a corpse into the grounds of a jinja is generally considered to be out of the question. Exceptions exist when the jinja honours the person whose corpse it is (for example, Dazaifu Tenmangū in Kyushu is built over the grave of Sugawara no Michizanë who, as Tenjin-sama, is the kami of the jinja), but even then they are exceptional.
However, the matsuri performed at this saidan are memorial services, attended by the living. In some cases, it is long enough since the death that, even on the strictest interpretation, nobody present is still suffering from kegarë. Further, the saidan is right next to the prayer hall. If the concern was with kegarë, one would think it should be further away, outside the precincts.
I suspect, although the article is not explicit about this, that the reason is much simpler.
The matsuri performed at the saidan do not honour the kami of the jinja. Instead, they honour the dead person. You cannot perform a matsuri for someone else in a kami’s sanctuary; that would be deeply impolite. The Shinto matsuri for the dead are not praying to kami to save the dead, as in Christian prayers for the dead, or for them to lead them to enlightenment, as in Buddhist services. Instead, the matsuri are honouring the dead directly, and asking them to protect their descendants. Thus, these matsuri are not performed in the sanctuaries of the kami.