Split-Level Home Design for Sloping Building Sites

Building on a sloping site presents both challenges and opportunities that flat sites do not. The grade change can be turned into an organizing principle for the design, allowing rooms to be arranged on multiple half-levels that follow the natural terrain rather than fighting it. A split-level approach uses the slope to create distinct zones within a single structure, with each zone at a different elevation. The Marniere residence in Ittre, Belgium, completed in December 2018, shows how a 415-square-meter home on a sloping 38-acre plot achieves this through careful embedding of the building volume into the terrain. The design process involved balancing the client’s desire for a rational, energy-efficient contemporary home with strict subdivision permit requirements, all while preserving existing trees and vegetation across the site. The relationship between the building flow of spaces and the site’s natural drainage patterns was a factor from the earliest planning stages.

Site Analysis and Setback Planning on Sloping Terrain

The first decision on a sloping site is where to place the building in relation to the street, the slope direction, and neighboring structures. The Marniere project is set back approximately 40 meters from the street, parallel to it. This distance is not arbitrary – it allows the main facade to take full advantage of the south-west orientation while preserving maximum trees and vegetation in the setback zone. Tree preservation on a building site of this size means less soil erosion during construction, better stormwater management, and immediate landscape maturity once the home is complete.

Open space requirements for ventilation in buildings are particularly relevant on sloping sites, where the grade change affects how air moves around the structure. A building set into a slope can block natural air drainage paths, creating pockets of still air on the downwind side. The 40-meter setback at Marniere provides enough distance for air to circulate freely around the building, preventing the microclimate issues that can occur when a large structure is placed too close to the slope edge.

Orientation and Solar Access

South-west orientation on a sloping site must account for how the slope angle affects solar access. A south-facing slope receives more solar radiation than flat land, while a north-facing slope receives less. The Marniere site’s south-west orientation provides good afternoon and evening sun exposure for the main living spaces on the first floor. The slope angle also affects how much of the building is visible from the street – the 40-meter setback means the building sits low enough in the landscape that it does not dominate the streetscape.

Working Within Height and Zoning Restrictions

The subdivision permit for the Marniere site imposed a maximum height under cornice of 4.5 meters. This restriction, common in subdivision developments from the 1960s, typically allows either a single-story building or a two-story building where the ground floor has limited ceiling height and the upper floor is partially engaged in the roof volume. The architects solved this by embedding the ground floor about 80 centimeters into the slope, effectively reducing the visible height of the building while maintaining comfortable ceiling heights inside.

Firms such as Judge Architectes et Associes have developed similar strategies for working within height restrictions on sloping sites, using partial basement excavation and split-level floor plates to maximize usable volume without exceeding zoning limits. The technique involves calculating the natural grade level at each point of the building footprint and adjusting the floor elevation so the finished floor-to-ceiling height remains compliant at the tallest exposed wall section.

Height Compliance Calculation Method

To comply with a 4.5-meter cornice height limit on a sloping site, measure the natural grade at each exterior wall location. The cornice height is measured from the lowest adjacent grade point. By embedding part of the structure into the slope – 80 cm in the Marniere case – the visible wall height decreases. The formula is simple: if the natural grade drops 80 cm across the building footprint, embedding the floor by the same amount means the exposed wall height at the downhill side equals the interior floor-to-ceiling height plus the floor structure depth, which can be kept under 4.5 meters even with generous ceiling heights of 2.7 meters.

Split-Level Structural Systems

The Marniere project consists of a main volume sloping across two levels and a secondary volume at a single level on the ground floor. This L-shaped configuration requires structural framing that can accommodate a change in floor elevation within the same building envelope. The main volume uses a continuous roof structure that spans both levels, while the secondary volume sits perpendicular to it at grade.

Foundation design on sloping sites must account for varying bearing pressures across the building footprint. On the uphill side, foundations may bear on undisturbed soil close to the surface, while on the downhill side, deeper footings or stepped foundations are needed to reach competent bearing strata. The ground water table also varies across the slope, which affects open well construction dewatering requirements during excavation. Dewatering on a sloping site typically involves interceptor drains installed along the uphill side of the excavation to capture groundwater before it reaches the foundation area.

Continuous Roof Framing Over Split Levels

A continuous roof over a split-level building presents framing challenges at the transition point where the floor steps up or down. The roof trusses or rafters must maintain a consistent pitch while the floor below changes elevation. This is typically handled by varying the stud height or using a balloon-framed wall at the split where the lower floor’s ceiling becomes the upper floor’s floor structure. The Marniere project uses a large continuous roof that meets the 4.5-meter cornice requirement at its lowest exposed edge while accommodating the split-level floor plate underneath.

Interior Spatial Organization Across Levels

Split-level floor plans organize rooms by function and privacy level. In the Marniere design, the ground floor of the main volume houses children’s rooms, a guest bedroom, and an office – the private and semi-private functions. The first floor contains the main living spaces – kitchen, dining, and living room – where the ceiling is freed from the height restriction by the roof volume above. This vertical separation of functions means that children’s activities and guest use happen on a different level from adult entertaining, reducing noise conflicts without requiring additional soundproofing.

The ground floor being partially embedded (80 cm below grade) means these rooms have higher thermal mass from the surrounding earth, providing natural temperature stabilization. However, below-grade rooms require careful moisture management. Why miter joints open up in wood trim and cabinetry in below-grade applications relates to the higher and more stable humidity levels in earth-sheltered spaces – wood moves less overall but the movement it does undergo is concentrated at glued joints that cannot accommodate the shift.

Moisture Protection for Embedded Levels

Walls that are partially or fully below grade require damp-proofing or waterproofing depending on the soil type and water table. A below-grade wall in sandy soil with good drainage may only need dampproofing (asphalt emulsion or cementitious coating), while clay soil or a high water table requires full waterproofing with drainage board and a perimeter drain system. The Marniere site’s significant natural relief provides good natural drainage, reducing the waterproofing requirements compared to a flat, poorly-draining site.

Carport and Secondary Volume Integration

The Marniere design places the carport on the left side of the house, tucked under the first-floor volume. This arrangement uses the same split-level strategy – the carport occupies the space below the elevated living floor, taking advantage of the height difference created by the slope. A secondary volume perpendicular to the main mass houses the entrance, garage, technical rooms, and laundry, creating a clear separation between the utility zone and the living zone.

This layout creates a covered outdoor parking area that does not require its own roof structure – the first floor above functions as the carport roof. The cost savings from combining structures in this way can be estimated at 15-25% compared to building a separate carport structure. The secondary volume’s technical room placement near the garage allows for easy access to mechanical systems without entering the main living area, which is convenient for service technicians and reduces disruption to the household. The principles of open-to-sky (OTS) construction apply to the space between the main volume and the secondary wing, where the gap creates a light well that brings natural illumination into the entrance zone without requiring additional windows on the exterior facade.

Thermal Performance in Split-Level Homes

Split-level homes present unique thermal challenges because the building envelope crosses the grade line, meaning part of the wall assembly is in contact with earth and part is exposed to outdoor air. The transition zone where the embedded wall becomes exposed above grade is a thermal bridge risk if not detailed properly. Insulation continuity at this junction requires the below-grade insulation to lap with the above-grade insulation by at least 300 mm to prevent a thermal shortcut.

The earth contact on the embedded level provides natural temperature moderation – soil temperatures at 1 meter depth remain between 7°C and 18°C year-round in temperate climates, reducing both heating and cooling loads for the rooms on that level. However, the above-grade portions of the same walls lose this benefit and must be insulated to the same standard as a fully exposed wall. Spray foam insulation excels in split-level construction because it can be applied continuously across the grade transition without gaps, filling irregular cavities where rigid board insulation would leave voids. Closed-cell foam on the above-grade sections provides air sealing and thermal resistance, while the below-grade sections benefit from XPS rigid board insulation that is not affected by soil moisture and hydrostatic pressure.

Energy Performance Comparison

A split-level home with a partially embedded ground floor typically uses 10-15% less energy for heating than an equivalent above-grade two-story home on the same site, due to the thermal mass and ground coupling of the lower level. The Marniere project was designed with energy consumption as a priority, using the PEB (Performance Energétique des Bâtiments) consultant Homeco to optimize the envelope and mechanical systems for the specific split-level configuration.