I find it difficult to imagine a situation where it would be necessary to make a Tzuras haPesach against a building and not rely on the wall behind it. Apparently you are referring to a situation when part of the Tzuras ha'Pesach is against the building, but it also includes a passageway under the Tzuras ha'Pesach, for instance when the Tzuras ha'Pesach is made in order to contain an opening that is greater than 20 high or 10 wide or Parutz Merubah, but the Tzuras ha'Pesach includes more than just the opening and is resting on the wall in which the breach exists. Alternatively, the Tzuras ha'Pesach begins against a wall, and extends out past the wall such that its outer post is away from the wall.

Here is a summary of some scenarios pertinent to your question, based on the Chazon Ish:

1) A wall which runs behind a Tzuras ha'Pesach on the outside of the enclosed area does not negate the Tzuras ha'Pesach

2) A wall which stands in front of a Tzuras ha'Pesach inside the enclosed area and is 4 Amos away from the Tzuras ha'Pesach does not negate a Tzuras ha'Pesach.

3) If there are less than 4 Amos between the Tzuras ha'Pesach and the wall is a kosher Mechitzah (i.e. 10 Tefachim high, does not sway in the wind, reaches within 3 Tefachim of the ground), then if the wall only partially fills the width of the Tzuras ha'Pesach but leaves a space of 4 Tefachim or more through which one can walk, the Tzuras ha'Pesach is valid.

4) If the space remaining in the width of the Tzuras ha'Pesach is less than 4 Tefachim then the Tzuras ha'Pesach would be invalid.

[From Hilchos Eruvin (Rav Elimelech Lange) Summary of Chazon Ish, Eruvin 70 #19]

Mordecai Kornfeld

Correct! Normally a wall cannot be on the immediate inside of the area enclosed by the Tzuras ha'Pesach, but it can be outside of the area enclosed by it.

In the case you mentioned, though, if the walls of the bungalows do not reach 3 Tefachim from the ground, they do not serve as Kosher Mechitzos and therefore they do not disturb the Tzuras ha'Pesach even if they are inside of it (see previous letter, #3).

If the Tzuras ha'Pesach spans two bungalows and there is an open space of at least 4 Tefachim between them then the Tzuras haPesach is valid in either case.

Kollel Iyun Hadaf

1) There are two problems with the Tzuras ha'Pesach on the bungalow: (a) The Lechi's of the Tzuras ha'Pesach are on top of an existing Mechitzah, and cannot be called the edges of a doorway; (b) the Tzuras ha'Pesach can only be seen on the outside of the area it encloses, and not on the inside (Nir'eh mi'Bachutz v'Lo mi'Bifnim, Eruvin 9b).

The Chazon Ish (OC 70:19) writes that if the Mechitzah in front of the Tzuras ha'Pesach does not have the Dinim of a Kosher Mechitzah then it does not invalidate the Tzuras ha'Pesach. The reason is as follows. If the Mechitzah is not valid as an Halachic Mechitzah then it does not separate the area inside the Mechitzah from the Tzuras ha'Pesach, and it is considered to be Nir'eh mi'Bifnim also, and not just mi'Bachutz. A Tzuras ha'Pesach should serve as an entrance to an area of at least 4 Tefachim, so if there are four Tefachim between the bungalow and the Tzuras ha'Pesach, it is also kosher even if the bungalow is a kosher Mechitzah.

2) Sefer Tikun Eruvin, by Rabbi Shulem N. Weiss, chapter 4:9, quotes the abovementioned Chazon Ish with reference to a Mechitzah in front of a Tzuras ha'Pesach. However, in #7 he writes with reference to a bungalow that is more than 3 Tefachim above the ground that there must be 4 Tefachim between the Tzuras ha'Pesach and the wall to be valid. He does not explain the apparent contradiction.

Perhaps, since the wall of the bungalow serves as a valid Mechitzah on the inside of the house, even though from the outside it does not reach within 3 Tefachim of the ground, it serves as a separation between the Tzuras ha'Pesach and the area beyond the house. (I heard from others, though, that it should be a valid Tzuras ha'Pesach in such a case.)

In general, I would recommend looking up your question in the excellent Sefer "Nesivos Shabbos" (Rav Yakov Blau, published by Mechon l'Hora'ah in Monsey), who discusses details of many everyday situations with regard to Eruvin. There's no substitute for bringing up the question with a local Talmid Chacham who is expert in these questions, though.

Best wishes,

-Ilan and Mordecai