Should the drainage system be considered in building planning? How to implement it?

Good morning everyone,

we will soon be building a semi-detached house in prefabricated construction (or having it built). The city’s development plan states that the rainwater must not infiltrate on the property but must be directed via drainage into two large infiltration pits at the edge of the new development area.

I have a few questions about this, which perhaps some of you can answer.

1) Who does this? The earthworks contractor? The prefabricated house company definitely will not do it, they only do the foundation slab + house. Or should I contact a landscaping contractor for this?

2) We are building a semi-detached house in Bauhaus style, meaning with a flat roof. There are four rainwater outlets: two at the back, two at the front. How do I get the water from the rear outlets to the front? Going past the foundation slab on the left is not possible because the neighbor’s house is there, on the right is also not possible because the strip foundations for the garage will go there. Under the foundation slab? Or through the foundation slab? I probably must not go too deep as otherwise there will be no slope left in the pipes towards the street.

3) How is the garden drainage supposed to work, do pipes need to be installed all over the garden?

4) And of course the most important question: What does this cost approximately? The plot is about 400 sqm, with a bit more than 200 sqm of garden.

EDIT: I just see that I landed in the wrong forum, maybe one of the mods can move this? :)

Good morning everyone,

we are soon building a semi-detached house in prefabricated construction (or having it built). The city’s development plan states that the rainwater must not seep away on the property but must be routed via drainage into two large infiltration pits at the edge of the new development area.

I have a few questions about this that maybe some of you can answer.

1) Who does this? The earthworks contractor? The prefabricated house company definitely won’t do it; they only do the foundation slab + house. Or should I contact a landscape gardener?

2) We are building a semi-detached house in Bauhaus style, meaning with a flat roof. There are four rainwater drains there. Two at the back, two at the front. How do I get the water from the rear drains to the front? Going left past the foundation slab is not possible because the neighboring house is there, going right is not possible because the strip foundations for the garage will be there. Under the foundation slab? Or through the foundation slab? I presumably can’t go too deep because otherwise there is no longer any slope in the pipes towards the street.

3) How is the garden drainage supposed to work, do pipes have to be laid everywhere in the garden?

4) And of course the most important question: How much does this roughly cost? The plot is around 400 sqm, of which a little over 200 sqm is garden.

EDIT: I just noticed I ended up in the wrong forum, maybe one of the mods can move this? :)

Good day, Drainage, infiltration trench etc. is also currently a topic for us, so here is my experience:

1) The drainage is the responsibility of the civil engineer. In our case, however, it is not the civil engineer who did the previous work on the property, because he is not allowed to work on public land. It would be wise if these works could be done in the course of the house connections/sewer works, so you need a civil engineer who is allowed to work both on your private property and on public land. Whether a landscape gardener can do this work, I cannot answer at this point.

2) I would guess that the pipes are laid along the house, under the foundations of the garage. The strip foundations at our place lie about 80-90 cm deep in the ground, so in your case they should be below that.

3) Unfortunately, I cannot give you reliable information about this, because we have to infiltrate via an infiltration trench on our own property. In my case, 1-2 pipes run underground along the house from the front to the back to carry the water into the infiltration trench.

4) The price depends greatly on the region and how many meters in total have to be dug up, but earthworks are very expensive; for us (a 10x10 meter house) it would cost around 4-5k. About 30 meters would have to be opened up.

1) Who does that? The earthworks contractor? The prefab house company definitely won’t do it, they only do the foundation slab + house. Or should I turn to a landscaping contractor?

Civil engineering company. It’s possible that the city has a company on standby for this. At least that was the case for us.

2) We are building a semi-detached house in Bauhaus style, meaning with a flat roof. There are four rain drains on it. Two at the back, two at the front. How do I get the water from the rear drains to the front? Going left around the foundation slab is not possible because the neighboring house is there, going right is not possible because that’s where the strip foundations for the garage will go. Under the foundation slab? Or through the foundation slab? I probably shouldn’t go too deep, otherwise there won’t be any gradient left in the pipes heading towards the street.

It would also be possible to go left to create a shared drain with the neighbor. That way you would only have to dig once from back to front. However, appropriate easements must be established for this. Otherwise, just under the garage.

3) How is garden drainage supposed to work, do pipes have to be laid everywhere in the garden?

Are you sure it only doesn’t apply to sealed surfaces?

4) And of course the most important question: How much does the whole thing cost roughly? The plot is around 400 sqm, of which a bit more than 200 sqm is garden.

You really can’t say here. It also depends on how high/low the plot is or the connection to public ground. If you have to lay the pipes 3m deep, it can quickly run into five figures.

1) Who does that? The earthworks contractor? The prefab house company definitely won't do it, they only do the foundation slab + house. Or should I turn to a landscaping contractor for that?

As writes, the civil engineer does that. If they lay the pipes up to a connection point at the street, that’s also fine; it doesn’t have to be the same company as the "development people" – these (water, sewage, telecommunications...) are better coordinated so that the street only needs to be dug up once if necessary (this also saves costs).

2) We are building a semi-detached house in the Bauhaus style, meaning with a flat roof. There are four rainwater drains on it. Two in the back, two in the front. How do I get the water from the rear drains to the front? Not possible on the left side next to the foundation slab because the neighboring house is there, not possible on the right because that’s where the strip foundations for the garage will go. Under the foundation slab? Or through the foundation slab? I probably can’t go too deep because otherwise there won’t be enough slope in the pipes towards the street.

Flat roof = Bauhaus style is comparable to a colorful painting with Expressionism. To get to the point: under the foundation slab is pragmatic and cost-effective. Not much slope is needed but it should of course be taken into account.

3) How should the garden drainage work, do pipes need to be laid everywhere in the garden?

I can hardly imagine that; usually it’s about draining water from sealed surfaces. The exact wording of the regulation will provide clarity here.

Thank you already for the detailed answers :)

I can hardly imagine; as a rule, it is about the drainage of water from sealed surfaces. The exact wording of the regulation will provide clarification here.

The development plan essentially states the following:

“Due to the soil conditions (loess loam and loess over terrace deposits), proper and professional infiltration can only take place in the deeper, sandy loess or the deeper-lying gravel sands (infiltration basins with gravel penetration)....

According to § 44 of the Water Act for the State of North Rhine-Westphalia (State Water Act – LWG), rainwater from plots that are developed for the first time must be disposed of in accordance with § 55 (2) Water Resources Act. According to the provisions of § 55 (2) Water Resources Act, rainwater should be infiltrated locally, percolated, or discharged directly or via a sewer system without mixing with wastewater into a body of water, provided that neither water law nor other public law regulations nor water management concerns oppose this.....

The infiltration of rainwater can, since the planning area is outside water protection zones, be carried out through two central infiltration systems. Decentralized systems are excluded. The detailed planning will be coordinated further with all parties involved.

As for rainwater infiltration, both above-ground infiltration areas as infiltration basins and underground infiltration facilities such as retention trenches are possible.”

How is this to be interpreted?

How is this to be interpreted?

This is ambiguous. Two central infiltration zones for the area can be established. Can is not must. Furthermore, reference is made to an agreement with the parties involved. So, off to the lower water authority and make an agreement. In §44 of the State Water Act, there is an economic efficiency clause. Read that in preparation for a conversation.