The living room, also called the residential hall, is exceptionally large, measuring 11 x 5.6m, and 4.3 m high. Adolf Loos used the contrast between the smaller dimensions of the entrance recess and the large room to create a moment of surprise that affects everyone entering. The extent of the room is augmented by the absence of a single longitudinal wall, these being reduced to load-bearing pillars which thus enable other parts of the house to be seen - the dining room on the mezzanine and the staircase to the upper floor. The living room contains seating concentrated into two groups along the shorter walls. The central space within the living room was left free by Loos, cleared of furnishing, which again strengthens the overall impression of size.
Loos' original spatial conception, known as Raumplan, is clearly visible in the living roof of Müller Villa: it has no doors, and its space is freely linked to the small entrance recess and via this to the entrance hall. Neither is the dining room separated by doors, this by contrast separated from the living room by a height differential. Finally, even the start of the stairway to the upper floor is not separated from the living room by a door. The one longitudinal wall is broken by the windows and door to the balcony. All of the windows in the living room are marked by their unusual, Japanising details of the crosspieces, partly veiled by the bright yellow silk curtains that appear throughout the villa with the exception of the family bedroom.
The colour scheme of the space is unusual. Loos had an exceptionally refined sense for colours, which made it possible for him to use even unusual colour combinations, as he chose appropriate colour values. Original colour combinations have already been seen during the tour of the interiors in the reception room, corridor and hall; in comparison with these, the scheme of the living room has a softer feel, even though green and purple or pink can be found in here in unusual conjunction. The rare, greeny-grey, lightly veined Cippolino de Saillon marble used to clad the walls and load-bearing pillars gives the main tone; the colour of the marble is offset by the golden yellow curtains. The dark, coffered ceiling of the dining room, visible between the pillars provides a counterweight. The ceiling of the living room itself is white, this whiteness providing a contrast to the colours of the oriental carpets.
To the left of the entrance, in the middle of the seating in the corner, is a large settee covered in dark purple velvet, standing between two marble-clad cubes. This blends in with the light tones of the floral decoration on the large, fully upholstered armchair. The colour of the wood of the other pieces of furniture complements the colour scale.
The short wall opposite contains a fireplace, within which is set a cast Neo-Classical relief depicting the fable of the stork and the fox; according to Mrs Müllerová’s records it comes from the fittings of the Palais Royal in Paris. The fireplace is faced with cut, white-pointed bricks, the earthy colour of which stands out in particular because of the proximity of the differently coloured marble facing. The brass radiator covers are of a colour comparable to the brass tabletop in front of the fireplace. The table has seating on both sides.
Their unusual shape links the pair of low, fully upholstered chairs that differ only in the colour of their velour covers: one is salmon pink and the other grey-green. Until recently Adolf Loos was credited with having designed these himself, but in 1989 Eva B. Ottillingerová drew attention to the fact that as early as 1900 the magazine Dekorative Kunst had carried photographs of similar chairs, citing L. Bernheimer as their design but describing the chair as an English import. The same researcher discovered another depiction of a similar chair in the magazine The Studio from 1901 – here it was part of an advertisement for its manufacturer, the London firm of Hampton & Sons, and is described as a ”New Shaped Easy Chair”, although the designer is not named – and a third in an undated, c.1900, price list for the Viennese firm of Friedrich Otto Schmidt, where it is called a ”Knieschwimmer-Fauteuil”. Ms Ottillingerová further notes that around 1900 Adolf Loos worked for Friedrich Otto Schmidt as a consultant. It is thus likely that Loos initiated the manufacture of chairs of this type, as he was a great admirer of English seating. Both of the chairs in the villa were in fact supplied by Friedrich Otto Schmidt in Vienna, while the other living room furniture was made by the Prague firm of Emil Gerstel and the fan-backed chairs by S.B.S. Brno.
It is interesting that almost every seat in the living room is a solitaire, this according with Loos' opinion that "everyone should find seating according to their mood and preference", It is also notable that the majority of the furniture is very low, and many people have difficulty rising from such low seats. Loos refused to accept such objections, saying that this "depends on habit. Anyone who travels in a low-slung automobile will be able to rise from my chairs."
Loos left the longitudinal walls almost without furniture. On one side he placed only two chairs, and on the other a commode with an undulating front and a rattan chair with upholstery of a floral pattern. Two aquaria were set into the parapet of the wall adjacent to the dining room, bringing the vibrancy of a changing picture into the interior. In the middle of this wall, on the landing of the stair leading to the upper floor, Loos placed a bronze bust of Antonín Müller, the builder's father and founder of the Müller–Kapsa company. The outstanding Czech sculptor Jan Štursa created the 1922 sculpture.
In all of his interiors, Müller's living room being no exception, Loos took the opportunity to use various types of lighting. He claimed that it is good to select lighting according to the needs of the moment or to the individual’s mood. This is reflected in the lighting installed in the villa's living area. The large ceiling lights made by the Prague firm of Franta Anýž & spol. were placed by Loos above the corner seats. The middle of the room has no central lighting, and is lit only by two bracket lights on the pillars between the windows, and shaded by the silk curtains. The corner seating with the purple settee could be lit by a ceramic table lamp with a parchment shade that stood on the cube at the side. There was intimate lighting by the fireplace - provided by the muted aquarium light or a fire in the hearth.
In the living room there are a total of five Oriental carpets. Loos selected these from Dr. Müller's extensive collection, and grouped them himself. The same approach was adopted in selecting paintings - these too he chose, grouped, and established the heights at which they were to be hung. Dr. Müller possessed an exceptional collection of paintings; to cite only a few examples, including three pictures by Adolf Kosárek, the same number by Julius Mařák, six by Antonín Slavíček and twelve by Jan Preisler.
Loos hung three pictures over the purple settee: in the middle Adolf Kosárek's Zimní večer ("Winter evening", oil on canvas, 1859), to the left Antonín Slavíček's Letní den ("Summer day", oil on canvas, 1906), and to the right one of Slavíček's last paintings, Letní krajína ("Summer landscape", oil on canvas, 1909). Initially, only a single painting was hung on the opposite wall - a large Alpine landscape by Antonín Hudeček - as shown by photographs taken soon after the villa was completed. Later photographs, however, apparently taken in 1932, show three paintings above the fireplace - Hudeček's Alpská krajina ("Alpine landscape", oil on canvas, 1916-17), now lost, remained in the middle, with a study of folk costume by Augustin Němejc (oil on canvas, 1905) on the right, and to the left a study of nude that cannot be identified from the photographs known. Because only the folk costume study could be traced it was decided that the reconstruction should take its inspiration from the first, older photograph showing the wall, and that only a single painting would be hung above the fireplace. In place of the lost Hudeček another picture was chosen from Dr. Müller's collection, of similar dimensions and genre: Otakar Lebed's Jihočeská krajina I. ("South Bohemian landscape I", oil on canvas, probably 1897). The last painting in the living room is Jan Preisler's Smutek ("Sorrow", oil on canvas, 1904), which hangs beneath the window from the lady's boudoir.