House extension: 2026 price, permits and joining it to the old house

An extension is the fastest way to gain an extra room, a bigger kitchen or a home office — without selling the house and moving. It has its specifics though: it is built hard against the old structure and in a small volume, which shows in both price and technology.

How much does an extension cost?

Prices excl. VAT. Yes — per m² an extension costs more than a new build (which starts at CZK 31,000/m²). The reason is simple: a small volume carries the full cost of site set-up, foundations, utility connections and the junction details with the old house.

The key technical topic: joining the old house

• Movement joint: a new extension settles differently from the old house in its first years — the structures are separated by a joint, otherwise renders and tiles crack.
• Foundations: new footings go beside the old ones to the same frost-free depth; undermining the old foundations is the most common DIY mistake.
• Continuity of insulation: damp-proofing and thermal insulation must continue seamlessly, or you get a thermal bridge and a damp joint.
• The opening into the old house: connecting rooms is a structural engineer's job — a lintel or frame in the load-bearing wall.

What to build the extension from

We choose the material by house and purpose: brick or Ytong for classic masonry houses, VELOX where speed and a monolith are wanted (walls in a few weeks), timber frame for light additions and weaker subsoils. The extension need not copy the house's material — it must connect to it correctly.

Permits and formalities

An extension changes the building footprint, so unlike minor structures it practically always needs a building-office permit and a design. Count on boundary setbacks, neighbours' participation in the procedure and, for larger extensions, utility operators' statements. We can handle the design and the paperwork.

How long it takes

Building a 20–40 m² extension typically takes 2–4 months. You can live in the house meanwhile — we leave the dusty break-through into the old house until the end.

An indicative price right away

In the calculator choose "New build → Extension", enter the area and see the budget immediately. On site we then assess foundations, the junction and statics — the visit and itemised quote are free.