A Peruvian pepper. On one side of the plot there was a very large and beautiful evergreen tree. Nearby, one of the neighbours’ fences was completely covered by a large bougainvillea and other varieties of climbing plants, a very pleasant corner with a lot of character, which was the only preserved part of the garden of an old nursery school. In that same area there were also some other trees, different bushes and a small palm tree. In the rest of the plot, there was almost nothing left, but it had some trees that jutted out from behind the fences of the neighbouring estates. The plot is located in a low-density neighbourhood, close to the centre of Castelldefels, where the vast majority of the neighbouring buildings are very old and, therefore, gardens with lots of vegetation have been consolidating there. The proposal tries to take advantage of and enhance the pre-existing elements on the plot, such as the dividing fences with the neighbours, the different perimeter plant species, types of climbers, groups of shrubby plants and above all a large tree, a Peruvian pepper, which is located in the southeast half of the plot. The house is articulated in relation to these perimeter pre-existences and especially around the large tree, developing only on the ground floor trying not to compete with the height of the tree, to always be below it in a human scale relationship and at the same time always looking for a direct and frank relationship with the garden and its perimeter. A garden that, instead of being an addition to the house, surrounds it and becomes an integral part of the experience, trying to make the garden and house form a single unit. Thus, the organisation of the house has a lot to do with the type of relationship that each part establishes with the garden. The important presence of the pepper tree and some other trees located on neighbouring plots, all of them evergreen, creates a lot of shade on the plot, leaving it too protected from the winter sun. After analysing the solar incidence on the house, we knew that it was difficult to capture sufficient radiation through the façades, so it was decided to capture the solar radiation through the roof. From this need the two most unique spaces in the house arise: the dining room and the study which, thanks to the precise and directed inclination of the roof, configure higher areas, crowned by large upper windows that allow natural light to be captured, solar radiation in winter and shading and ventilation in summer. These spaces, apart from their bioclimatic potential, are the ones that structure all the others, organising circulations and establishing more complex relationships between them. Around the dining room, the fireplace and the kitchen, the laundry room, a bathroom and the piano room are grouped, while surrounding the study we find the bedrooms and a second bathroom. From these spaces, we can visually relate to the outside through the roof or indirectly through the other rooms, creating chains of different qualities and intensities of light that shade and degrade the passage from inside to outside and that throughout the day, and depending on the solar incidence, they alternately single out the rooms, favouring the diversity of environments. Thanks to the rotation of one of the rooms, a gap appears that interrupts the continuity of the façade, breaks the system of central courtyards and allows the garden to sneak inside the house. It is through these interruptions that it is accessed and therefore the central spaces become transitional between the outside and the inside, patios and halls at the same time. This twist, the abundance of natural light that enters through the upper skylights, added to the rougher and more natural ceramic flooring just gives them a more outdoor, patio or garden extension atmosphere than the other rooms of the house that are more conventional, both in their proportions, the treatment of light and their finishes. The dining room and the study preserve the finishes and textures of the façade and, therefore, despite being interior, they remind us or transport us to the outside, while in the perimeter rooms all the finishes are continuous coverings much more typical of an interior The double cross organisation of the building, apart from making the centrality of the main spaces possible, also serves to reinforce the autonomy of each room with respect to the house as a whole, so that all the spaces can establish a very close relationship with the garden. Its compact layout, with a lot of façade, turns the rooms into boxes surrounded by a garden that at the same time surrounds them, also separates them from each other and they only communicate with each other through the central spaces which, due to their bioclimatic characteristics, in many moments also behave as an interior extension of the garden. From the outside, the cross arrangement succeeds in fragmenting the perimeter garden into narrower and more collected spaces as if they were outside spaces. The spaces that result from the tension between the geometry of the house and the perimeter of the plot are like a second house on the outside. In fact, the best space in the house is outside the building, between the walls of the boxes and under the large crown of the existing tree, as if the whole house had undergone the tree and in its shadow. The walls that make up the boxes are always thick because they solve the structure of the building and its thermal behavior in a single layer so that we cannot differentiate the façades from the structure of the interior enclosures. It is a single wall that is folded and unfolded creating the distribution from the same constructive solution. A wall thick enough to be self-stable, dense enough to support the slabs but at the same time light enough to be the thermal insulation of the house. A single-sheet solution, from a 30 cm thick piece of lightened clay ceramic block (Poroton-Planziegel T-10, from Wienerberger) that has a very low thermal transmittance, while also providing the house of much thermal inertia. The combination of insulation and thermal inertia in the same layer allows passively regulating and optimising the hygrothermal exchange between the interior and the exterior and guarantees very high levels of comfort throughout the year. The wall is organised by horizontal strips, in those areas where the thermal requirements are not so strict, combining different models of lighter clay blocks to respond case by case to the different constructive requirements based on their formats and their thermal and bearing properties. Above this wall, the roof is flat and only the roofs of the central spaces stick out, which incline and stand out from the wall structure with a very light external materiality, typical of a firefly, and which on the contrary cover the interior space with a heavy concrete slab, with a lot of thermal inertia that accentuates the lightness of the practicable closures of the skylight. The slightly reflective finish of the slab achieves a better diffusion of natural light while at certain times it reflects the green of the exterior vegetation achieving a certain dematerialisation of the roof and making the large tree more present inside the house.