Digging a basement under an existing house: underpinning and cost

Can you build a basement under a house that already stands?

Yes, provided the ground can carry the load, the water table sits below the future floor level and the planning parameters allow it. The method: the existing foundations are underpinned section by section (recalce), the interior is excavated under the protection of new walls and the slab is cast. You need a geotechnical study, a design by a qualified technician, a permit and settlement monitoring. On the coast the limit is usually the water, not the structure.

A villa from the 1970s-80s on shallow strip footings is among the more favourable cases: simple walls and clear loads. More difficult is a house on old rubble masonry, on fill, or on a slope. The decision is made not by how the house looks but from three documents: the geotechnical report, a survey of the foundations (trial pits beside the walls) and the municipal plan.

If something does not add up, the basement can be replaced by an extension, a semi-basement with a light well, or a garage with a car lift. We cover that in our guide on extending a home.

What is underpinning and how is it done?

Underpinning (recalce) means deepening and strengthening the existing support of the house without moving the house. The most common method is bataches: short sections under the foundation are dug alternately, usually around a metre wide, a concrete support is cast to the required depth, left to gain strength, and the next section follows. At every moment the house stands on completed sections.

The second method is micropiles. Holes are drilled beside the foundation or through it, grout with reinforcement is injected and the load is carried deeper. It helps when there is little room for machinery or the ground is weak. The geotechnical report decides between the two.

The order of work in practice: survey and trial pits, geotechnical study, design and calculation, permit, temporary props, underpinning in sections in the calculated sequence, excavation inside, new basement walls, slab, waterproofing and drainage. Settlement is monitored throughout: tell-tales on the walls, levelling and a log. If movement exceeds the limit, work stops. It is not a formality; it is the only way to learn about a crack before it shows on the facade.

How does groundwater decide whether a basement is feasible?

What decides it is the position of the basement floor relative to the water table. Above the water, the basement is built with conventional waterproofing and drainage; at water level or below, it is built as a sealed tank designed for water pressure. Pumping continuously to keep it dry is not a good idea: it lowers the water level around the site and affects neighbours. Whether pumping is allowed is checked with the municipality and Catalonia's water administration.

CTE DB-HS 1 classes the presence of water as low (the floor is above the water table), medium (at the same level or less than two metres below) and high (two metres or more below). That, together with the permeability of the soil, sets the waterproofing grade. For drainage solutions the document provides for wells or chambers with two pumps and an automatic device so that pumping runs permanently. The pumps must stay powered during a power cut; see our guide on backup power.

What to ask the designer for: the water level from the geotechnical boreholes, seasonal variation (in Barcelona and on the coast it depends on rainfall and proximity to streams and sea), the adopted waterproofing grade and the drainage solution. If those terms do not appear in the project, that is a warning sign.

What about neighbours, loads and spoil removal?

Neighbours. If the house stands on the boundary or shares a wall, a record of the condition of adjoining buildings is drawn up before work starts, with photos and tell-tales on cracks. Underpinning can only be done within your own plot; you cannot go under the neighbour's land. Take out the contractor's liability insurance and make sure the contract is clear about who answers for damage.

Loads. The new basement walls and slab redistribute the load on the soil. That is calculated in advance: what the house carries, what the new walls carry, how the slab behaves when the water rises. A pool or a large tree next to the house changes the picture, so tell the designer.

Spoil. A worked example: a 100 m2 basement 3 metres deep produces about 300 m3 of soil in the ground, about 390 m3 once loosened, which means roughly 25-30 truck trips at 15 m3. In a narrow urbanización street or a courtyard with no access this is the most delicate part of the job: it takes permits, time windows, small machinery and a conveyor instead of an excavator. Ask how the contractor will remove the spoil and where it will be taken; it is a separate line in the quote.

Does a basement need its own permit?

A separate application is not usually needed: you apply for the ordinary major-works licence (obra mayor), with a project by a technician and a site manager, and the basement is part of that project. Timelines and fees are set by each municipality and are confirmed at the ayuntamiento. What is actually checked: the permitted use of the basement (parking, storage, plant rooms; habitable use only if light, ventilation and height requirements are met), whether below-ground floor area counts towards buildable area under the municipal plan, and what share of the plot the underground part may occupy.

On the coast, coastal law is added: in the protection strip the Ley de Costas applies and limits works on existing buildings. See our guide on the Ley de Costas. In Collserola and other protected areas, also check the rules on trees and slopes.

Do not begin earthworks before the licence. Work without a licence can be halted and fined, and regularising a basement under a house after the fact is far harder than getting the licence first.

How much does a basement under a villa cost?

A general per-metre figure means little before the geotechnical report, and no serious contractor will give one. In practice a basement under a finished house costs noticeably more than the same area in a new build: the work proceeds in sections, with small machinery, access limits, and every section has to cure.

Six things set the price. Depth and area. The ground (rock, clay, fill). Water and the waterproofing grade, including pump chambers. The underpinning method: bataches or micropiles. Access and spoil removal. The basement's fit-out and services: ventilation, dehumidification, electrics, stairs, a light well. On top come the design, the geotechnical study, site supervision, insurance and permit fees.

In the quote ask for separate lines: geotechnics and survey, design and supervision, underpinning, earthworks and removal, walls and slab, waterproofing and drainage, services, finishes, contingency. If there is no contingency and everything shows as one "basement" figure, you do not have a quote.

How does BCN YARD approach a basement under an existing house?

We do not start with a quote but with a site visit: we look at the house, the access, the neighbours and the list of questions for the ground. The design and geotechnics are done by independent specialists, and we run the works: underpinning, earthworks, structure, waterproofing, services, finishes. One person is responsible for the site, in Russian and Spanish, with staged payment under contract. If the geotechnical report shows the basement is not worth it, we say so up front and propose another route.