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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/948,025 filed Sep. 23, 2004, now U.S. Pat. No. 7,540,120, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/505,401 filed Sep. 23, 2003, both of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to a ‘luxury’ class of residential apartment structures which emulate the features of detached dwellings. When applied to vertically superimposed apartment units found in high-rise developments, the combination of features offered by the present invention is usually limited to penthouse locations, which are customarily less constrained in height and footprint. The present design makes penthouse features available to all floors of such structures, without sacrificing the energy efficiency inherent in a simplified building envelope, or the economy and constructability of a conventional high-rise structural system. Efficient fabrication is inherent in the economy of scale of such structures, and energy efficiency is inherent in their low surface-to-volume ratio. 
     BACKGROUND OF THE INVENTION 
     High-rise development is favored in an increasingly crowded and environmentally challenged world. The higher density afforded by such structures reduces the per capita footprint and consequent impermeable surfaces which cause flooding, pollution of storm water, high albedo effects, and the congestion engendered by sprawling development due to longer travel distances, and consequent fuel consumption, air pollution and other effects of a ‘carbon footprint’. 
     The present invention addresses itself to that mobile class of homeowners whose means and inclinations most commonly result in sprawl. It offers a viable alternative to the private detached dwelling that achieves comparable amenities together with the environmental efficiencies of high-rise development. 
     Two attributes of the design cannot be claimed as program objectives in efficient apartment planning, but make a positive contribution to a visually complete environment: The stairs permit vantage points that enrich the visual experience, and a modicum of physical rigor that has come to be valued in an increasingly sedentary world. The corridors serve the private zones of the dwelling by imparting a sense of seclusion, and refuge from the boisterous communal functions, as well as providing gallery space for personal expression. 
     The ‘complete visual experience’ fulfills a cognitive need inclining humans to three dimensional complexity in the spatial environment. It exercises the faculty of depth perception enabled by stereoscopic vision—a unique adaptive response to the primordial arboreal habitus. This complexity therefore brings into the apartment an element that enables the space to be comfortably occupied for longer periods. It is believed that this is essential to the definition of habitability. 
     SUMMARY OF THE INVENTION 
     A residential apartment unit arranged on multiple levels capable of providing the foregoing attributes of a detached dwelling to all units of a building. The apartment unit is contained within a demising envelope whose shape fulfills at least four objectives: 
     1. Distributing the enclosed volume of a multi-unit apartment building meaningfully by moderating the height of each space within the apartments according to its function and proportionately to its plan dimensions. Within the floor area limits imposed by zoning laws, this distribution of volume enables utilization of an asset that is ordinarily wasted by apartment ‘flat’ designs with ceilings of conventional height. Such designs typically consume less building volume than the full bulk envelope to which zoning laws entitle them; 
     2. Nesting of vertically stacked volumes to conserve building height; 
     3. Horizontal alignment of such stacked units to consolidate and convey vertically continuous services, and accommodate shafts, and chutes; and 
     4. Complementary pairs of such units form a rectangular footprint that offers a simplified boundary for vertically continuous supporting structure. The units generally extend across a centrally located public corridor to opposite facades of the building. Each public corridor provides access to a row of such apartment pairs only on entry levels, which increases the vertical interval between corridors to over two stories, thus minimizing uninhabitable floor area in the building. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Essential features are illustrated as preferred embodiments of the invention in the following drawings, in which like reference numbers and letters indicate the same or similar elements: 
         FIG. 1  is an exploded axonometric view of all four levels of a single apartment  10 A sharing a public corridor with the second apartment of a pair  10 B. The stairs, utility channels, room titles, balconies, partition and fixture layouts are identical in each, as those of a preferred embodiment of the design. 
         FIG. 2  is an enlarged view of the typical interior stair and support wall with utility channel shown in  FIG. 1 . The stair is that of a preferred embodiment of the design. 
         FIGS. 3A ,  3 B,  3 C and  3 D are portions of the Floor Plan of a building with identical apartments of a preferred embodiment, taken at different elevations along the stepped line of Section  3 - 3  shown in  FIG. 5 . The hatched areas distinguish separate apartments. 
         FIGS. 4A ,  4 B, and  4 C are Cross-sections of a preferred embodiment of apartments within a portion of a building taken along Sections  4 A,  4 B,  4 C respectively, shown in  FIG. 5 . The hatched area distinguishes the extent of a single apartment. A legend designated “R” indicates the number of risers above (+) and below (−) the entry level (0.0) in the interior stair of a preferred embodiment. The extent of these levels in plan is reflected in  FIGS. 3A ,  3 B,  3 C, and  3 D. “FC” designates a preferred position for a suspended fan coil unit with hydropic connections for temperature control of the major communal spaces within the apartment, served by ducts. 
         FIG. 5  is a Longitudinal Section of a portion of a building with identical apartments of a preferred embodiment taken along Section  5  as shown in  FIGS. 3A ,  3 B,  3 C,  3 D,  4 A,  4 B,  4 C. The hatched area indicates the extent of a single apartment. Stairs of a preferred embodiment are shown schematically. Partitions and certain bearing walls are omitted for clarity. 
         FIG. 6  is a rendered perspective view of a furnished apartment in a preferred embodiment seen from the level of the entry landing, looking across Module B toward the window and exterior balcony. A perspective line drawing of the same view is provided below the rendering for orientation. 
         FIG. 7  is a rendered perspective view of a furnished apartment in a preferred embodiment seen from the interior balcony  69  of Module D, looking across Module B toward the window and exterior balcony. A perspective line drawing of the same view is provided below the rendering for orientation. 
         FIG. 8  is a rendered perspective view of a furnished apartment in a preferred embodiment seen from the floor  38  of Module B, looking toward the interior wall containing the apartment entry, the interior stair, and the parapet of the interior balcony. A perspective line drawing of the same view is provided below the rendering for orientation. 
         FIG. 9  is a perspective line drawing of a preferred embodiment showing the split-level relationship of Modules A and B in the context of other modules. The exterior balcony is omitted for clarity. 
         FIG. 10  is an enlargement of  FIG. 3B , a furnished partial Floor Plan in a preferred embodiment, showing Modules A and B, with an exterior balcony  40 . 
         FIG. 11  is an enlargement of  FIG. 3C , a furnished partial Floor Plan in a preferred embodiment, showing Modules C and D, with an interior balcony area  69 . 
         FIG. 12  is an axonometric volume study that demonstrates the geometric relationship between the principal demising elements and the spaces they contain. Portions of certain demising walls and partitions are omitted to clarify the spatial pattern schematically. 
         FIG. 13  shows an alternate interior stair arrangement for a typical apartment unit. Its implications are shown by combining a partial Floor Plan  13 A, with a Cross-section  13 B, and a Longitudinal Section  13 C arranged in cabinet drawing format. 
         FIG. 14  shows a second alternate interior stair arrangement for a typical apartment unit. Its implications are shown by combining a partial Floor Plan  14 A, with a Cross-section  14 B, and a Longitudinal Section  14 C arranged in cabinet drawing format. 
         FIG. 15  shows a third alternate interior stair arrangement for a typical apartment unit. Its implications are shown by combining a partial Floor Plan  15 A, with a Cross-section  15 B, and a Longitudinal Section  15 C arranged in cabinet drawing format. 
         FIG. 16  is an exploded axonometric view of apartment units showing their demised envelopes diagrammatically as geometric solids. These assemble as components of a building that resolves into a rectangular solid. Structural and utility elements are omitted for clarity. The shape of the unit depicted in this view is predicated on the preferred embodiment of the internal stair design. 
         FIG. 17  is similar to  FIG. 16  but seen from below wherein component units are presented as shaded geometric solids. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is comprised of an arrangement of spaces in an apartment unit organized on basically four vertically displaced floors. The floors are connected by a stair system that may, in a preferred embodiment, incorporate a support wall containing a utility channel for vertically continuous services. 
     Referring first to  FIG. 1 , a first apartment unit  10 A is entered from a centrally located public corridor  25 , nominally coplanar with the first apartment floor level  32  corresponding to Module A. The floor level  32  extends from the apartment entry  34  to the building façade on the same side of the public corridor  25 . The ceiling plane of the floor level  32  of Module A is designated  33  (see  FIG. 9 ). 
     Module A would typically contain the customary functions associated with an entry, including but not limited to a Wardrobe, Utility or Storage Closet, Powder Room, Kitchen, Dining Room. Alternative and/or additional functions may of course be designed without deviating from the scope and spirit of the invention. 
     The height of Module A and the public corridor  25  is greater than a minimal story height to accommodate an interstitial space for horizontal transfer of utilities within the module and the public corridor  25 , above the plane of the finished ceiling. These utilities typically include, but are not limited to, pitched sanitary and condensate drains, air conditioning ductwork, and toilet, dryer and kitchen power exhausts. The additional headroom in the public corridor  25  imparts the formality expected of this function, while the functions within the larger communal spaces of Module A similarly benefit from additional ostensible height in proportion to their greater floor dimensions and desired formality. 
     A landing extension of floor level  32  into Module B affords access to the interior stair system leading to upper and lower levels of the apartment.  FIG. 2  shows a preferred embodiment of the interior stair system. Descending the steps  36  from floor  32  accesses the floor  38  of Module B a split-level distance, or less than one-half story, below Module A. The floor  38  extends from the public corridor wall to the same building façade as that of landing  32 L. The floor  38  includes a landing  38 L adjacent to the steps  36 . 
     Module B would typically contain the customary functions of the lower portion of a split-level communal space including but not limited to a Living Room and Den or Study. Alternative and/or additional functions may of course be designed without deviating from the scope and spirit of the invention. 
     The height of Module B, in a preferred embodiment, is substantially equal to the height of Module A plus approximately twice the difference in elevation between Modules A and B. In addition, as a minimum, sufficient headroom clearance is provided above and beneath a platform  52  of the interior stair to allow passage, where required by the desired stair geometry, and in conformity with applicable building codes (see  FIGS. 1 and 2 ). 
     Descending steps  42  from the floor  38  in Module B, the floor  44  of Module C is accessed less than a full story below the floor  38  in Module B. The floor  44  extends from the bottom riser of the steps  42 , coplanar with an interior corridor  29 , beneath the public corridor  25  to the opposite façade of the building from that bounding Modules A and B (see  FIG. 1 ). The floor  44  includes a landing  44 L adjacent the steps  42 . 
     Further, the floor  44  is excised to allow headroom clearance required by the upper portion of the interior stair system within Module B of the apartment immediately below. The floor  44 , in a preferred embodiment, is further excised to allow clearance for an atrium space within the Module B of the apartment immediately below. The dimensions of interior corridor  29  result from the two foregoing openings in the floor  44 . 
     Module C, in a preferred embodiment, would typically contain the functions of a Master Bedroom suite, including Wardrobe, Master Bath and Bedroom. While its function(s) and dimensions are intimate enough in character to justify a lower ceiling height and privacy from other spaces, its formality relative to those spaces warrants proximity to the Living Room, whose social functions it may share. Alternative and/or additional functions may of course be designed without deviating from the scope and spirit of the invention. 
     The height of Module C may be the minimum story height. In addition, as a minimum, sufficient headroom clearance is provided beneath the public corridor  25  to allow passage on steps  42  to the interior corridor  29 . 
     Ascending steps  50  from the stair landing at floor level  32 , past stair platform  52  and above (see  FIG. 2 ), accesses the floor  68  of Module D above the public corridor  25 . The steps  60  lead to a landing  68 L of the floor  68 . The floor  68  of Module D, including communal area  69  in addition to the landing  68 L, extends from the apartment entry wall to the same façade as that bounding Module C. 
     Module D, in a preferred embodiment, would typically contain the functions associated with one or more minor bedrooms, including Wardrobes, Toilet, and Bedrooms. Alternative and/or additional functions may of course be designed without deviating from the scope and spirit of the invention. A communal area  69  on floor plane  68  falls behind a parapet as an interior balcony overlooking Module B. 
     The height of Module D varies. That portion over the floor that includes the Bedroom and Toilet functions may be of minimum story height. The height over the two combined areas excised from the plane of floor  44  of Module C immediately above, may extend upward an additional story to the same ceiling plane as that of the aforementioned Module C immediately above, as shown in  FIGS. 4B and 4C . A suspended ceiling may lower the ceiling plane in this location to provide interstitial space for horizontal services and recessed lighting. 
     The line of sight from the interior balcony of communal area  69  to the floor of Module B is controlled by the distance of the line of the atrium excision in Module C of the apartment above, measured from the parapet wall of the interior balcony of the communal area  69  of Module D. 
     The foregoing describes a preferred embodiment of a typical apartment unit  10 A. The irregular demising envelope of this unit is complemented by an identical unit  10 B rotated 180 degrees in plan about the central public corridor  25 , as shown in  FIG. 1 . Module A of apartment  10 A is thus directly across the public corridor  25  from Module B of apartment  10 B. The entrance to apartment  10 B, through its Module A, is thus on the opposite side of the public corridor  25  from that of apartment  10 A, and diagonally opposite the Module A accessing apartment  10 A. 
     The footprint of the resulting apartment pair is thus a rectangle, providing simplified boundaries for vertically continuous supporting walls  27  extending from one façade to the other, perpendicular to, and penetrated by the public corridor  25 . 
     The public corridor&#39;s efficiency thus benefits from a double-loaded configuration, while its preferred centrality and vertical alignment with those of other vertical segments above and below, allows direct access to vertical shafts, chutes and service risers, minimizing non-habitable space. Structural efficiency also benefits from vertical alignment of the public corridors since building codes commonly assign higher design load standards to these than to the occupied portions of residential floors. Although the public corridor  25  may be in a schematically or nominally central location between apartments  10 A and  10 B, its vertical alignment with the public corridors of other segments above and below is more essential to the design. The building may be designed such that apartments on either side of the public corridor  25  vary in dimensions perpendicular to the public corridor  25 . 
     Structural support, in a preferred embodiment, is assisted by a vertically continuous load bearing wall  71  located between Modules A and B to receive one side of the floors  32  and  38  not supported by demising walls  27 . Wall  71 , in a preferred embodiment, is partially open within the apartment to allow Modules A and B to communicate visually over a parapet, and to allow passage between these modules at the landing  32 L, as shown in  FIG. 1 , or equivalent as shown in  FIGS. 13A-15C , or other. The wall  71  of apartment  10 A extends nominally parallel to walls  27  within the apartment, and continues across the public corridor  25  to the corresponding second apartment of the pair  10 B, coplanar with wall  71  of the first apartment of the pair  10 A, from one building façade to the other on the opposite side of the building. 
     Apartment pairs may be added to the public corridor in mirrored juxtaposition, such that Module A of apartment  10 A on one side of the corridor is adjacent to the Module A of the next apartment on the same side of the corridor, and Module B is similarly adjacent to Module B of the next apartment. The pattern is repeated on the opposite side of the public corridor  25 , starting from the opposing unit, apartment  10 B, as shown in  FIG. 3B . The major room functions in a preferred embodiment shown in  FIGS. 3A-5  are as follows: K=Kitchen, DR=Dining Room, L=Living Room, BR=Bedroom, MBR=Master Bedroom. 
     Apartment pairs may be stacked vertically in the same orientation to each unit as those flanking it horizontally, as described above. In this arrangement, the following juxtapositions result: 
     Module A of apartment  10 A of the first vertical segment falls above Module C of apartment  10 B of the first vertical segment, 
     Module B of apartment  10 A of the first vertical segment falls above Module D of apartment  10 B of the second vertical segment, 
     Module C of apartment  10 A of the first vertical segment falls above Module B of apartment  10 B of the second vertical segment, 
     Module D of apartment  10 A of the first vertical segment falls above Module A of apartment  10 B of the first vertical segment. 
     When stacked vertically, preferably the public corridors  25  align vertically with one another. However, it is possible that the position of the public corridors  25  in stacked vertical segments may be horizontally offset from one another. 
     In a preferred embodiment, utilizing the interior stair support wall  12  as a utility channel, the horizontal alignments of stacked apartments provide a straight vertical path  14  through each of two such embodiment, these channels are exposed on four sides only where the stair is exposed in Module B, and concealed partly or entirely by partitions within the other modules. The stair support wall  12  of the preferred embodiment includes two major surfaces  16  and two lateral faces  20 . 
     In a preferred embodiment, exterior balconies  40  are appended to floor  38  and/or floor  32  extending beyond the fenestration line of Module B and/or Module A. When applied to these locations, they occur at an interval of nominally five stories, as shown in  FIGS. 4A ,  4 B,  4 C. This increases the azimuth by which each balcony receives direct sunlight for the benefit of gardens, while imparting the ambience of an open terrace. The absence of shading by overhangs at the ceiling plane of the communal spaces promotes penetration of daylight into the deeper recesses of these larger spaces, as by clerestory fenestration. In the event that site orientation prohibits sunlit balconies at Modules A and B, then Module C serving the Master Bedrooms is the next most suitable level. 
       FIG. 16  shows an exploded view of an apartment building including several pairs of apartments  10 A and  10 B in each of two vertical segments arranged one on top of the other. Each pair of apartments includes apartments  10 A and  10 B and part of the public corridor  25  extending longitudinally alongside the apartments  10 A and  10 B and providing access to both apartments  10 A and  10 B from opposite sides of the public corridor  25 . As described above, apartment  10 A is rotated 180 degrees in plan in relation to apartment  10 B about the public corridor  25  and each of the apartments  10 A and  10 B includes at least four floor levels suitable for habitation (see  FIG. 1 ). A floor level of Module A is substantially co-planar with a floor level of the public corridor  25 . Also, in each of the apartments  10 A and  10 B, the floor level of Module A and the floor level of Module B extend from a plane of a wall containing an entry  34  into the apartment to a building façade on a first side of the public corridor  25  on which the entry  34  from the public corridor  25  into the apartment is situated (see  FIG. 3B ). A floor level of Module C of each apartment  10 A and  10 B extends beneath the public corridor  25  to a building facade on a side of the public corridor  25  opposite to the side to which the respective modules A and B extend. The floor level of Module D of each apartment  10 A and  10 B extends above the public corridor  25  to the building facade on the same side of the public corridor as that to which the respective Module C extends. A stair system is arranged in each apartment  10 A and  10 B to connect the floor levels therein (see, for example,  FIGS. 1 and 2 ). 
     Although preferred embodiments of the present invention have been described and illustrated herein, and some alternatives are cited, the universal applicability of many of its precepts make it adaptable to many circumstances. It is therefore obvious that modifications in proportions, dimensions and quantity of units, as well as partitioning and furniture will be needed to respond to requirements imposed by building codes and program objectives. Many such modifications, too numerous to describe herein, can be made without departing from the principles of the invention. For example, although the arrangement of spaces comprising an apartment unit described above is organized on basically four vertically displaced floor planes, it is conceivable to provide fewer or more floor planes, e.g., one of the floor planes may include a bi-level structure to thereby provide the apartment unit with five floor planes. This additional floor plane does not alter the fundamental novelties of the invention and thus the essence of the invention will remain the same.

Summary:
A system of spaces including a residential apartment unit in a multi-unit building arranged on multiple levels that provides the following attributes to all units in the building: Central corridor access to a middle level of each unit; through-unit feature with exposures at opposite sides of the building; a variety of ceiling heights, each appropriate to the function and formality of its respective space, which nest to conserve building height and bulk; and internal balconies and exposed interior stairs enabling vertically displaced spaces to interlock and communicate visually. The resulting unit is contained within a demising envelope whose shape, when rotated 180 degrees about a central public corridor serving a second such unit, forms a complimentary pair. The combined shape of each such pair of units enables it to be stacked vertically and abutted laterally into a simple rectangular solid that allows vertical continuity for optimum efficiency of structural and mechanical support systems.