市街化調整区域にある小さな住宅群の一角。実家の敷地を分筆して夫婦と2人の子供のための住宅を計画する。母屋の住環境への配慮、世帯ごとのプライバシー確保、夫婦専用の駐車場を用意するなど実家との関係性に配慮した要望が目立った。関係性を隔てるのではなく、各世帯のプライバシーを守りつつ適度な距離感を保って暮らすことを望んでいた。
実家の敷地は北西の角地で、接道している北側道路が西へ向かって鈍角に折れ曲がり下っていく坂道に面しており、東道路面と南隣地面が最高で3.5mほどの擁壁で支えられた高台になっている。与えられた計画地は母屋の庭先部分で、間口10.5m奥行き20.7mのやや細長い形状をしており、隣地と擁壁に囲まれて接道していない。実際に訪れてみると、周囲との高低差が舞台上のような晒された感覚を強調するせいか、どこか落ち着かない印象を受けた。
調査を進めてみると、いずれの擁壁も既存不適格扱いとなった。敷地全体が盛土で造成されているため地盤も頼りなく、杭などを用いてその場しのぎの対処法で建築することは不可能ではないが、いずれにしろ接道させるため擁壁に手を加えなければならない。西側道路面の擁壁は巨石を積み上げた空積み擁壁で、一部が母屋の基礎を直接受けており、ひび割れて沈下し始めている箇所も見受けられるほど危険な状態にあった。構造家の立ち会いのもと、地盤の安全性を担保するための措置が必須であると判断された。
老朽化した擁壁の危険性は社会問題に発展している。擁壁を維持管理する責任は所有者が負うことになるが、その社会的認識やコスト面から誰しもが適切に対処できるものではないにもかかわらず、擁壁を多用して宅地を量産してきた時代背景に起因する。擁壁を残し管理していくことに施主は不安を感じていた。これを機会に土地を擁壁から解放し、そのうえで与えられた環境条件と真摯に向き合ってみることのほうが、より進歩的で豊かさに満ちた暮らしの場を創造できるのではないか。こうして、できるだけ擁壁に頼らずに建築する方法を目指すことになった。
検討を進める中で、幼少期の経験を思い出していた。山や原っぱで遊んでいるとき、凹地（おうち）を見つけるとちょっとした特等席のような気分で身を潜めた。拠り所のない自然の中で、包まれることに潜在的な安心感を覚えていたのだろう。竪穴式住居や穴居という住居形式が世界各地で発明され、また「母なる大地」という共通言語が示すように、土中の恒温性能を求めるだけでなく、太古の人々は大地に潜る・出るという行為（母神性）に重きを置いていたという民族学的見解がある。諸説はあっても、子供ながらに感じていた安心感はその起源と無関係ではないように思う。洞窟のように強い閉塞感はかえって不安を煽ることもあるが、凹みと呼べるくらいの按配で、身を潜めることも顔を覗かせることも選択できるおおらかな包容力のある場所になったら心地良さそうだ。
計画地全体を安全角の法面として掘り下げることで既存擁壁をできる限り減築し、東側道路面の実家を支える残った空積み擁壁を補強する役割と土留めを兼ねたRC擁壁だけ新たに築造した。斜面、迫る家々と繁る竹藪によってぽっかりと凹地のような場所が生まれ、そこに身を潜めるように建築する。間口の狭い敷地において法面処理だけで高低差を解消するのは難しく、基礎が法下を少しだけ受け止めるかたちを取った。それが結果として大地と建築の関係性をより親密にし、凹地の中に身を置くような臨場感を高めた。
北東側の法面に影を落とさないよう高さを抑え、太陽高度から勾配を導いた片流れ屋根とすることで、明るい斜面の庭を実現した。斜面に向かって積極的に開くことで実家との関わりを残しながら、豊かな朝日や斜面からの反射光を取り込むという副次的効果にも期待している。南西面は隣家が近接しているため少し背伸びするように最高高さを設定し、設えた高窓が移ろう陽を捉える。屋根の下中央に雛壇状の共用空間を配置することで、気分に応じて居場所を選ぶことができる。ホールに身を置けば外の気配に触れたり空を仰ぐことができる伸びやかな開放性を与え、斜面の庭と屋根がLDKへの視線を遮り密やかな安心感をもたらす。
凹地の家が、自然の恵みと住人の暮らしをおおらかに包容する受け皿になることを願っている。（境原桃太、境原彩香）
A residence that gives a sense of security and presence nestled in the earth
A corner of a small group of houses in an urbanization control area. The family home site was subdivided to plan a house for a couple and their two children. Many requests were made for consideration of the relationship with the family home, including consideration of the living environment of the main house, ensuring the privacy of each household, and providing a parking space exclusively for the couple. Rather than separating the relationship, the couple wanted to maintain a moderate sense of distance while preserving the privacy of each household.
The family home site, nestled in the northwest corner of a small urbanization control area, presents a unique set of challenges. The site faces a slope where the north road turns sharply to the west and descends, while the east road and the south neighbor surface are elevated, supported by a 3.5m high retaining wall. The site, which is the garden part of the main house, is long and narrow, with a frontage of 10.5 m and a depth of 20.7 m. It is surrounded by the neighboring land and a retaining wall and is not connected to the road. During our site visit, we observed a sense of unease, possibly due to the elevation difference from the surrounding area, which created a feeling of exposure akin to being on a stage.
Upon further investigation, we found that both retaining walls were treated as existing nonconforming. Since the entire site was built on a barrier, the ground was unreliable, and although it was not impossible to construct the building by using temporary measures such as piles, the retaining walls would have to be modified in any case to make them accessible to the road. The retaining wall on the west side of the road was an empty retaining wall made of piled-up boulders, and part of it directly supported the foundation of the main building, which was in such a dangerous condition that it was cracking and beginning to settle. In the presence of a structural engineer, it was determined that measures to ensure the safety of the ground were essential.
The danger of aging retaining walls has developed into a social problem. The responsibility for maintaining and managing retaining walls lies with the owner, but this is due to the historical background of mass production of housing lots with extensive use of retaining walls, even though not everyone can adequately deal with them due to their social recognition and cost. The owners were concerned about leaving the retaining walls in place and managing them. Taking this opportunity to free the land from the retaining walls and then sincerely dealing with the given environmental conditions would be a better way to create a more progressive and affluent place to live. Thus, they decided to pursue a method of building without relying on retaining walls as much as possible.
During my examination, I was reminded of a childhood experience. When I was playing in the mountains or the fields, if I found a recess (home), I would hide away, feeling like I was in a particular place. I felt a latent sense of security in being enveloped by nature, where I had no hiding place. As pit dwellings and cave dwellings were invented in many parts of the world, and as the common language “mother earth” indicates, there is an ethnographic view that people in prehistoric times not only sought the constant temperature performance of the earth but also placed importance on the act of diving into and out of the earth (matriarchal nature). Despite the various theories, the sense of security felt as a child seems unrelated to its origins. A strong sense of enclosure like that of a cave can be unsettling, but it would be comfortable to create a place with a generous receptiveness that allows people to choose whether to hide or show their faces in a proportion called a recess.
The existing retaining wall was reduced as much as possible by excavating the entire project site as a safe slope, and only an RC retaining wall was newly constructed to reinforce the remaining piled-up retaining wall supporting the family house on the east side of the road and to serve as an earth retaining wall. The houses and bamboo thickets on the slope create a hollow-like space where the house is built as if it were hiding. Solving the height difference only by treating the slope on a narrow site is difficult, so the foundation is designed to catch a little under the slope. As a result, the relationship between the earth and the architecture was made more intimate, and a sense of presence, as if one were in a hollow, was heightened.
The height of the roof is kept low so as not to cast shadows on the northeast slope, and a single-slope roof with the slope guided by the solar elevation creates a brightly lit slope garden. By actively opening toward the slope, the garden is expected to have the secondary effect of bringing in abundant morning sunlight and reflected light from the slope while retaining a relationship with the family home. The southwest side of the house has a neighbor’s house nearby, so the maximum height was set a little higher than usual, and the high windows caught the shifting sunlight. The common space is placed in the center under the roof so that residents can choose where to stay according to their mood. The sloping garden and the roof provide a sense of security by blocking the view of the LDK.
We hope that the house on the concave land will be a receptacle that generously embraces the blessings of nature and the lives of its residents. (Tota Sakaibara, Ayaka Sakaibara)
【凹地の家】
所在地：愛知県知多郡
用途：戸建住宅
クライアント：個人
竣工：2021年
設計：境原建築設計事務所
担当：境原桃太、境原彩香
構造設計：山内義章（ストラリズモ一級建築士事務所）
施工：誠和建設株式会社
撮影：境原建築設計事務所
工事種別：新築
構造：木造
規模：地上2階
敷地面積：211.92m²
建築面積：74.52m²
延床面積：129.84m²
設計期間：2017.11-2020.11
施工期間：2020.11-2021.10
【House in Recessed Area】
Location: Chita-gun, Aichi, Japan
Principal use: Residence
Client: Individual
Completion: 2021
Architects: Sakaibara Architects
Design team: Tota Sakaibara, Ayaka Sakaibara
Structure Design: Yoshiaki Yamauchi / Stra Rizmo
Construction: Seiwa Kensetsu
Photographs: Sakaibara Architects
Construction type: New building
Main structure: Wood
Building scale: 2 stories
Site area: 211.92m²
Building area: 74.52m²
Total floor area: 129.84m²
Design term: 2017.11-2020.11
Construction term: 2020.11-2021.10
