ᐅ Which is more suitable for the site conditions: a basement or a slab foundation?
Created on: 28 Jan 2024 19:38
T
TobsBAU
Hello everyone,
I am new to the forum and have purchased a plot of land from the municipality in a new development area, where a single-family house is planned to be built within the next 1.5 years (1 or 2 stories).
For some time now, I have been struggling with the site conditions (slight slope and below street level) and the associated question of whether a basement might actually be the smarter/more cost-effective (or at least cost-neutral) solution in this situation. To answer this question properly, I will probably need quotes from civil contractors and, above all, a geotechnical soil report. Regarding the soil report, I seem to be going in circles because all geotechnical engineers want very precise information about the planned construction, which in turn depends on the decision between a basement or slab-on-grade foundation.
I am hoping to get advice on the following questions:
All prefab house suppliers, etc., I have contacted so far have really not addressed these conditions. They just ask for a budget range and then assume a flat fee of €25,000 for earthworks, etc. I suspect this will not be enough, and I want to make these costs tangible for my financial planning! When the basement topic comes up, even architects generally say a basement is always expensive. If the budget is tight, better to do without a basement...
Here are some details and premises I want to share with you:
Thank you very much, and I look forward to your suggestions and advice!
I am new to the forum and have purchased a plot of land from the municipality in a new development area, where a single-family house is planned to be built within the next 1.5 years (1 or 2 stories).
For some time now, I have been struggling with the site conditions (slight slope and below street level) and the associated question of whether a basement might actually be the smarter/more cost-effective (or at least cost-neutral) solution in this situation. To answer this question properly, I will probably need quotes from civil contractors and, above all, a geotechnical soil report. Regarding the soil report, I seem to be going in circles because all geotechnical engineers want very precise information about the planned construction, which in turn depends on the decision between a basement or slab-on-grade foundation.
I am hoping to get advice on the following questions:
- Is it possible, when building with a basement, to use the excavated soil to fill the plot up to street level (no costly disposal required)?
- What advice can you give me about the soil report (the exact future construction is not yet fixed)?
- I am torn between the following options (which do you think is more sensible?):
- Build with a basement and fill the plot to street level using the excavated soil (hold back the northeast property boundary with a 1 m high retaining wall). Allow a partial slope to create a daylight basement room facing northeast, which could possibly be used as an office. The utility room would naturally also be in the basement, but I would prefer to avoid a sump pump system if possible.
- Fill the plot (up to street level or slightly below) and build with a slab-on-grade foundation (hold back the northeast property boundary with a 1 m high retaining wall). However, I am concerned about the costs for fill material and necessary compaction. The utility room would be on the ground floor or upper floor, no sump pump system, but probably with a backwater valve.
All prefab house suppliers, etc., I have contacted so far have really not addressed these conditions. They just ask for a budget range and then assume a flat fee of €25,000 for earthworks, etc. I suspect this will not be enough, and I want to make these costs tangible for my financial planning! When the basement topic comes up, even architects generally say a basement is always expensive. If the budget is tight, better to do without a basement...
Here are some details and premises I want to share with you:
- The plot is below street level. The adjacent neighboring plot (southeast) has been raised up to street level (top of finished floor level +156.14 m).
- Because of the filling up to street level, no backwater valve was needed there. On the opposite plot (northwest, behind the sidewalk), the fill was not raised as high, so a backwater valve was required. If I build with a basement, I would probably have to plan for a sump pump system even if I fill up to street level.
- According to the development plan, a retaining wall up to one meter high may be built to compensate for the slope; beyond that, slopes with an angle of 30° must be used (see attachment). Of course, both neighboring properties have taken advantage of this and installed suitable L-shaped retaining walls on the northeast side.
- Basically, I would not have necessarily filled the plot, but since the southeast side was raised and that is where most of the natural light comes from, I feel almost compelled to at least partially fill.
- Unlike the neighboring development, I am considering positioning the house (gable roof) with the gable perpendicular to the street (see sketch). This would improve lighting and usability of the plot.
- There is a castle to the northeast at some distance, which should also be taken into account when planning the house (view connection).
- The plot is 464 m² (about 5,000 sq ft) and located 800 m (0.5 miles) from a river (about 15 m (50 feet) above the water surface, no flood risk). Additional waterproofing measures for a basement are to be expected (most likely a waterproof concrete shell). A neighbor who built with a basement had to seal it from the outside (apparently afterwards).
- The development plan is generally quite generous: 2 full floors allowed, max ridge height = 6.5 m (21 ft 4 in), max eaves height = 10.5 m (34 ft 5 in), floor area ratio: 0.4
- Surveying has been done (see attachment)
Thank you very much, and I look forward to your suggestions and advice!
K a t j a schrieb:
I somehow feel the terrace lacks privacy because of the house placement. Is this the only way? I can’t think of a better solution right away. A hedge?
I would position the house further back and avoid placing a garage on the south and west sides. Personally, I prefer to keep a courtyard area that offers more options for window design, but maybe I’m just overcomplicating things.
hanghaus2023 schrieb:
Here is my suggestion. Why is the house axis oriented perpendicular to the slope?
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H
hanghaus202330 Jan 2024 14:3111ant schrieb:
Why is the house axis perpendicular to the slope?After filling, this is no longer really a slope. In my opinion, this is reasonable here since the building includes a basement.hanghaus2023 schrieb:
In my opinion, this is justified here because a basement is being built. You misunderstood: the original poster is considering whether the slope allows for a basement or if they should build without one. See the headline 😉
hanghaus2023 schrieb:
After the backfilling, this is no longer a slope. In my opinion, this is a reasonable case here since a basement is being planned. ypg schrieb:
You misunderstood: the original poster is considering whether the slope allows for a basement or if they should build without one. See the title 😉 Here, even if it’s only a slight slope, with extensive backfilling, there is no longer a slope.
I can only recommend to the original poster my (not just local) posts on the topic of basement regulations and the basement question, and I clearly see a partial basement or a moderate split-level design with a courtyard or an adaptation to the terrain like @Nordlys.
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Thank you for the numerous additional contributions.
When building the garage, some privacy will be provided (see attachment, garage top level planned at +158.75 m (521 ft)). The door on the neighbor’s house shown in the picture is not the main entrance, but an entrance from the garage. The two upper windows belong to the bathroom and bedroom. You can expect some view of the terrace from there.
The two neighboring houses have their terraces facing northeast (towards the view), which differs from the concept currently under discussion (for the reasons mentioned).
Would you then build the partial basement exclusively as a utility basement in the front area (towards the street)?
Regarding the split level, I could barely find any information (additional costs, etc.). That would make sense, for example, in the northeast corner of the floor plan or (see attachment)?
hanghaus2023 schrieb:As already mentioned, this is exactly the question (basement or slab), especially considering total costs. My approach now would be to commission a soil investigation and request quotes from excavation contractors to offer various options:
Thanks for the further documents. I also agree with the architects' opinion. Reference height is 155.77 m + 5 m (16 feet) * 2% = 155.87 m (509 ft) finished floor level.
With the eaves height and ridge height, almost everything is possible here.
Since you are building with a basement, you probably have some leftover material that you can use for filling.
I would talk to the neighbor about allowing the filling up to your property boundary with a 2:1 slope. That benefits them because they can build the garage, and you save fill material.
Please check whether a carport can be placed closer to the street. Otherwise, you will have the same problem with the path as your southern neighbor.
- Fill and compact the plot almost completely up to 155 to 156 m (511 to 512 ft) finished floor level.
- Partially fill (building footprint and terrace, optionally with a 1 m (3 ft) retaining wall) and compact up to finished floor level.
- Excavate the basement pit, use the excavated material for backfilling on the plot, etc.
K a t j a schrieb:The placement is definitely a compromise between: the view of the castle/forest to the northeast, usability of the plot towards the south, roof orientation for photovoltaics, and so on.
I somehow feel there is a lack of privacy on the terrace due to the house placement. Can it only be done this way? I can’t think of a better solution right away.
When building the garage, some privacy will be provided (see attachment, garage top level planned at +158.75 m (521 ft)). The door on the neighbor’s house shown in the picture is not the main entrance, but an entrance from the garage. The two upper windows belong to the bathroom and bedroom. You can expect some view of the terrace from there.
The two neighboring houses have their terraces facing northeast (towards the view), which differs from the concept currently under discussion (for the reasons mentioned).
11ant schrieb:What do you mean by "terrain-adaptive design like Nordlys"?
Even if there is only a slight slope, after generously filling it no money is left. I can only recommend my (not just local) posts regarding basement rules and the basement question, and I clearly see a partial basement or a moderate split-level with a courtyard or a terrain-adaptive design like @Nordlys.
Would you then build the partial basement exclusively as a utility basement in the front area (towards the street)?
Regarding the split level, I could barely find any information (additional costs, etc.). That would make sense, for example, in the northeast corner of the floor plan or (see attachment)?
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