Design Workshop: Eco retreat meets design reality
Rob Norman from design firm Symbiosphere unpacks the design challenges faced by a couple planning to build an eco retreat in Queensland’s Gold Coast hinterland. In the process, he crafts a design for their proposed treehouse cabins.
We present an excerpt below; read the full design response with floor plans in Sanctuary 25.
Jessica and Michael have a dream to build an eco retreat on their Gold Coast hinterland property in the foothills of Springbrook National Park. They want to share the natural beauty of the land with others by building five new mid- to high-end holiday cabins around their existing home. Their plan is that these cabins connect to the site and make the most of views over forested land to the ocean. They would like each cabin to have its own private space and be surrounded by trees.
Jessica and Michael’s current house and shed sit on a cleared piece of flat land at a high point of the property that is almost surrounded by tall eucalypt trees. It has impressive views to the Surfers Paradise skyline and ocean beyond. There are short-range views into the nearby forest, with a valley to the south also offering leafy outlooks.
The pair’s first step towards realising their dream has been to prepare the site and replant cleared areas. On land that was once a banana farm, they have planted natives and plan to uncover a spring-fed stream previously buried in sediment runoff.
The project is still in its early stages and Jessica and Michael have many ideas they’re considering. They have obtained some initial concept plans for the cabins and have picked out five sites. Three sites are located in the narrow valley to the west of the central building (sites 1, 2 and 3). These sites fall quite steeply in different directions (east, south and west) and each have beautiful panoramic views across the valley and longer views to the hills to the south. Sites 1, 2 and 5 are currently intended to be earth-covered houses and sites 3 and 4 treehouses. Sites 4 and 5 are both on an east-facing slope with prime views to the coast in the northeast.
At this early stage in the project there are still many design challenges to be worked through. Gold Coast-based sustainable designer Rob Norman casts an eye over their concept cabin plans and advises them on the significant design work still to do to bring their dream to reality.
- Design cabins that either resemble a treehouse or are earth-covered.
- Connect these cabins to the site and views.
- Make treehouse cabins that sit as high as possible but are still accessible. Trees shouldn’t be used as structural supports.
- Create cabins that keep cool in summer using passive solar design, not air conditioners.
- Create cabin designs that adapt to the specific orientation, topography and surrounding vegetation of each site.
- Include a composting toilet and bore water or rainwater tank in each cabin.
Designing for the site
Jessica and Michael’s property is in a subtropical climate with hot, humid summers, often with high rainfall through until about April. During this season buildings need to operate like a tropical house with extensive shading, very good cross-ventilation and large eave overhangs to allow ventilation when it is raining. Winters in the area are comfortable during the day but can get quite cool at night (5–10 °C) and buildings without winter solar gain and good insulation will be uncomfortably cold without artificial heating.
All the proposed cabin sites have reasonable access to northern sun in winter for passive solar design and solar hot water. However, scattered shading from trees would seriously affect the efficiency and output of photovoltaic panels at individual cabin locations. The different orientations of each cabin will also make it challenging to find a single design that maximises winter solar gain, minimises summer sun and maintains views, particularly where views are to the west.
At this stage the project is still very conceptual, and I would expect there will be some significant changes before the designs are fully resolved, including planning, structure, materials, and services. The siting of the cabins is well thought through with regard to access, views and location (perhaps with the exception of site 5 due to privacy and sound issues arising from its close proximity to the central building).
For the initial concept cabin designs to perform well climatically, each one will need to work in a more site-specific way, particularly with respect to differing orientations and slopes. All of the cabin designs have the potential to work quite effectively on a north-facing slope as they have plenty of glass and adequate sun shading to the east and west, however none of these sites actually faces north. More work needs to be done to make a repeatable design that is flexible enough to adapt to a variety of sites, including those where there are conflicts between the slope or views and northern orientation.
Natural light, glazing and sun control
With their one-room-deep floor plans and generous amounts of glass along one side, plenty of natural light should reach all spaces in the initial designs. I would take advantage of the privacy offered by each site to include larger bathroom windows, as this will give them a more generous quality.
Each design has a large glazed wall with a few smaller windows to other aspects (no other windows for the earth-covered house). This would be an appropriate amount of glass if each cabin faced north but not appropriate if they face other directions, which is the case here as they will be difficult to shade in summer and will act like a heat sink in winter. With its proposed south-facing orientation, the earth-covered house will be the most difficult design to bring winter sun into and with its large thermal mass could remain uncomfortably cool in winter.
As some of the sites have their main views to the east or west, sun control becomes a significant issue during the hotter months. Operable (openable) shading systems such as hinged or sliding screens are a possible solution to control direct sunlight. They can be closed to block summer sun and opened to views when necessary and in winter.
Cross ventilation shouldn’t be a problem in the proposed treehouses but the earth-covered house has limited capacity for cross ventilation and in the humid, rainy season condensation could occur at the back of the cabin, potentially causing mould. As a minimum there should be ventilation through the roof along the back wall of the cabin.
As the property could generate a sustainable supply of firewood, the use of wood heaters is probably the best form of artificial heating. The cabins are compact, so an efficient fireplace (not an open fireplace) should heat the space effectively irrespective of the cabin’s location.
Servicing is another aspect of the designs that should be addressed. Composting toilets require a sizeable piece of infrastructure below them and this is very difficult in an in-ground house and potentially unattractive in an elevated treehouse. Along with rainwater tanks and solar hot water, these elements need to be considered as an integral part of the building design rather than as add-ons.
Of the proposed designs, the compact treehouse has the most potential to create a wonderful accommodation experience with the simplest and most economical building. It can take advantage of the steep sloping sites to put a living platform six metres or more above the ground and yet be accessed from further up the slope by a relatively simple ramp/bridge structure, maintaining access for those with limited mobility. The two-storey treehouse could create interesting spaces and would be achievable. This design may be more expensive to construct than the earth-covered house, however the underground house will most likely have to contend with more (and riskier) issues, including waterproofing, control of overland flow from up the slope and impacts on slope stability.