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!
11ant schrieb:
Prefab house suppliers because of the common misconception that they are the first choice for more accurate final price estimates?
The foundation slab must be seriously included in the offer simply because it is part of the scope of work of the (shell construction) general contractor. Without the foundation slab, everything is meaningless; it is literally THE basis. Additionally, the top of the foundation slab is the root of all critical interfaces.
This is an intelligence test to check your ability to think beyond the obvious, with a little bit of lubricant added for safety.
Your plot of land is the same for every contractor. Always commission the survey yourself. I deliberately advised a broad screening of the entire building site area (close enough to the corners to assess soil homogeneity) because it makes no sense to commission a new survey for every specific house footprint ("retract that order!").
Although I was not directly addressed by the question, I am happy to share (and did so before in "A Housebuilding Roadmap, also for you: the HOAI phase model!") the generally valid answers: always a self-chosen independent architect, NEVER!!!! phases 1-4, phase 9 is also complete nonsense for an owner-occupied single-family house, but rather:
1. Phases 1 and 2 (called "Module A" in the housebuilding roadmap)
2. Resting period (usually with a decision point; here I would remove all timber specialists, so only ask 3 or 4 masons)
3. Phases 3 to 8 (= Modules B and C) preferably with the same architect (note: 3 + 5 = 8).
And general contractor again, my eternal refrain: "generally welcome, but not without a proper tender process." The homeowner mistake of assuming from the balanced scorecard that they can safely treat it as a black box is an expensive popular fad. Hello 11ant, where can one buy your housebuilding roadmap?
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hanghaus20231 Feb 2024 20:08I already showed it in post #38 how a prefab house builder can do it. Just a little leveling and that's it. This is possible even without a basement.
hanghaus2023 schrieb:
I showed in post #38 how a prefab house manufacturer can handle it too. Just a bit of grading, and that’s it. Even without a basement. Your plans are really great, but in my opinion they still can’t replace a local expert. Issues like water supply, wastewater, connections, heavy rain, slope grading, usability of excavated material, paths, stairs, choice of construction materials, assessment of the soil report – and much more – all have to be considered in the decision-making process before pouring the foundation slab. That is still far from what a prefab house can cover.
K a t j a schrieb:
If it were mine, I would definitely start with an architect and go up to phase 2. And very important: only then proceed to the resting phase (and normally also the preliminary decisions). The resting phase (which I have explained in detail in the house-building schedule series) lasts about six weeks, and only after that do you continue differently depending on the construction method. The preliminary decisions are mostly made passively, all within the resting phase.
K a t j a schrieb:
Here, I would first check whether the project has potential. The feedback from the preliminary decisions also shows whether the house size needs to be adjusted. If, after the resting phase, you have a good feeling about the chemistry with the architect, you continue working with them.
K a t j a schrieb:
If so, I would proceed with them up to phase 5. Overall, including service phase 8, but here actually within the next architect contract installment up to service phase 5 (if – only for a slab-on-grade house – timber construction might also be an option, then initially only service phase 3 instead of the full module B).
K a t j a schrieb:
You can also mix both and, for example, continue with the general contractor after phase 2. I would do at least service phase 3 with the architect, and only start with the general contractor at service phase 4. Service phase 3 is not for mere draftsmen.
K a t j a schrieb:
But everyone has to figure that out for themselves. That said, as mentioned, you should always involve the general contractor after, and not instead of a tender process.
Julloef schrieb:
Hello 11ant, where can your house-building schedule be purchased? You can get your personalized copy at the same place where the general version is freely available online – on "Bauen jetzt" or by following the search phrase from post #47 including the quotation marks.
You are also welcome to present your building project here in a dedicated thread and already receive detailed advice in the open consultation. I also recommend to my clients to participate in the forum: many eyes see more.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
Hello everyone, I’m back with the results of the exploratory drilling (a ground investigation report cannot or will not be prepared for me without concrete floor plan designs). To me, the findings seem unspectacular. What is your conclusion regarding the basement (waterproof concrete shell) versus slab-on-grade foundation?
Thank you and best regards
Thank you and best regards
H
hanghaus202323 Feb 2024 09:25Everything is category 4 earthquake resistance, and with no groundwater present, it seems to me that any type of construction should be possible without issues.
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