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Expand the basement and gain space in the basement
Expand the basement and gain space in the basement

Video: Expand the basement and gain space in the basement

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Video: Basement Dig Out, Lowering Your Basement 2023, January

At some point you get tired of the lumber rooms in the basement. Wants order. Needs space. Would like to incorporate comfort. Do it: You need to know this if you want to expand, convert and use the basement and thus want to upgrade the basement.

Living space is becoming scarcer and more expensive, so why not expand the basement? And thus create space for the study or children's room, the long-desired sauna? Maybe you can even rent a separate apartment.

First things first - be sure to have a building permit. A change of use is required if the classic cellar is converted into living space. If you build without, and the building authority gets on your way, it will be expensive.

In order to get a permit, you must pay particular attention to the number of storey areas (GFZ). This number regulates the maximum number of square meters of floor space (external dimensions of the building) permitted on the property. Example: GFZ 0.7 with a plot size of 600 m 2 means that you may not have built more than 420 m 2 floor area.

If you have already used your property well, the GFZ could become a problem. Each federal state has its own state building code. This determines how high the clear room height and how large the windows must be. The requirements for fire protection are also changing. A second escape route must be available, in most cases an external basement staircase. And the utility cellar must be separated from the living room with an F 90 fire protection wall (offers 90 minutes resistance to fire) and a fire protection door.

Submit building application for basement expansion

To be able to submit the building application, you need a planner who can submit it to the building authority. At the same time, an architect will help you with a clever room and lighting concept. The planner creates the drawings for the craftsmen and makes an invitation to tender for the work involved. He also coordinates the trades on the construction site and ensures that the conversions run smoothly.

The craftsmen should also be considered. The order books of the individual trades are currently well filled, making spontaneous, short-term expansion almost impossible. Plan long-term and ask all craftsmen in good time so that there are no delays on the construction site.


Anyone planning a basement expansion should take care of the craftsmen in good time.

Photo: Kadmy / Fotolia

Heating, water and electricity when expanding the cellar

If the sewage system is above the deepest drain in the basement, you must install a waste water lifting system. This pumps the water over the backflow level and also prevents the wastewater from flowing back and flooding its basement.

Install your own water meter for a tenant to offset the amount actually used. The apartment must also be equipped by the energy supplier with a separate circuit and fuse box.


If you want to rent a separate apartment in the basement, you should install a water meter to check the amount of water actually used.

Photo: flashpics / Fotolia

In most cases there are no radiators in the basement, but luckily the existing heating usually creates the additional power easily. Since the clear room height often does not allow for installation height for underfloor heating and screed, classic radiators are recommended - or possibly wall heating.

Remember that the basement remains cold on an uninsulated floor slab. You should also insulate the ceiling from the ground floor so that the impact sound in the basement remains within the tolerance range.

The air in the basement tends to stand still more, and opening the windows doesn't always make sense either, but is rather harmful. Controlled ventilation, matched to the behavior of the residents, improves the indoor climate enormously and ensures that mold has no chance.

Ventilate cellars better in winter than in summer

You ventilate the basement exactly the opposite of what you think. Because especially in basement ventilation, the layperson's ignorance often causes moisture damage. Therefore the rule is: Not in summer. If you want to ventilate the musty basement with fresh air in summer, get damp walls instead. This is due to the so-called fridge effect: if we take a well-chilled bottle out of the fridge in the summer, water drops immediately settle on the surface. Because warm air stores a lot of moisture - and if it hits a cold surface (for example the bottle), condensation automatically falls out. This is exactly what happens when we ventilate the basement on hot days: The humidity in the warm air is reflected on the cold basement walls. Instead of fresh air, we pump moisture (and mold) into the basement.

Kondenswasser am Fenster
Kondenswasser am Fenster

Do not ventilate the cellar in summer, otherwise there is a risk of moisture damage.

Photo: iStock / nadisja

So: Keep the windows in the basement closed on muggy summer days and air them in winter. If there are living rooms in the basement and you also wash, dry and iron in the basement, ventilate in the early morning when the air is still cool. The most effective way to remove existing humidity from the basement is to heat it up for several hours in winter (the heat pulls the moisture out of the wall) and then let in icy (as dry) air. A hygrometer helps you to keep an eye on everything.

Choice of materials for the expansion of the basement

If the rooms in the basement are used for living, you should energetically strengthen the outer walls with insulation boards. These must meet two requirements: be capillary active and non-flammable. Calcium silicate sheets or capillary-active mineral foam sheets permanently protect against moisture and prevent mold growth.

Otherwise a true all-purpose solution, plasterboard has lost nothing when expanding the basement. The risk is too great that the plates will absorb the moisture and thus create the breeding ground for mold.

Kellerdecke streichen
Kellerdecke streichen

The color used should be open to diffusion. Therefore paint the basement walls with a silicate or lime paint.

Photo: Fotolia / Schulzfoto

Sealing slurry is a liquid cement-plastic mixture that is applied with a puff, usually in two layers. It serves to seal surfaces against the ingress of water. The sludge should dry out very slowly.

Restoration plaster is a particularly open-pored plaster that increases the drying surface and allows the wall to dry out faster. It also binds dissolved salts so that they cannot penetrate the wall surface.

Only plaster that does not contain gypsum should be used for finishing the basement wall. Because plaster absorbs water and swells. The color used It should be open to diffusion, so it should be able to “breathe”. Therefore paint the basement walls with a silicate or lime paint.

Seal the basement and dry it

Many cellars in the old building are more like a wetland. The basement wall and floor slab are exposed to tougher conditions and, in addition to the general earth moisture, must also resist oppressive and high standing groundwater. In many older buildings there is a need for action to make the basement usable as a storage space and above all as a living space. There are various options for sealing the basement:

External sealing

The most sensible and best method to keep the damp wall permanently dry. Unfortunately, this is also the most complex, expensive and not possible method for all buildings. For this purpose, the basement walls must be completely exposed, i.e. dug down to the foundation base. A circumferential drainage should be laid here to drain backwater. In addition to the usual bituminous or mineral waterproofing (in combination with perimeter insulation), you can also use an ancient Roman method - a clay-clay mixture through which no water can penetrate.

Horizontal seal

A horizontal layer injected below the basement ceiling prevents the capillary moisture from rising into the masonry. This form of sealing is highly recommended to protect the walls.


In this method, an acrylic gel or a resin is introduced into the wall or into the adjacent soil via boreholes - it hardens and completely seals the masonry in its substance.

Inner seal

If it is impossible to protect the house from the ingress of water from the outside, a multi-layer system is applied to the inner wall and floor. The moisture remains in the outer wall. The seal prevents moisture from penetrating the new surface. This does not solve the problem of the damp wall - but it helps to get the problem under control and to be able to use the basement to its full extent - and the basement shines in new splendor.

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