--- title: "Extension Cost — What Does It Cost to Build an Extension?" canonical_url: https://fremtidenstegnestue.dk/en/knowledge/articles/extension-cost markdown_url: https://fremtidenstegnestue.dk/en/knowledge/articles/extension-cost.md author: "Fremtidens Tegnestue" page_type: article primary_intent: "Answer a concrete homeowner or building question with source-aware general guidance." primary_keyword: extension audience: "Homeowners, studios and agents researching a concrete building topic." summary: "What does an extension to your house cost? See prices per m² for garden rooms, ground floor extensions and first floor additions — and what drives the price." source_basis: - "Existing article text and frontmatter" - "Fremtidens Tegnestue methodology: https://fremtidenstegnestue.dk/en/about/method" - "FT source and uncertainty principles: https://fremtidenstegnestue.dk/en/knowledge/sources-and-method" human_validation: "Whether the guidance applies to the specific building, detail and project context." responsibility_line: "AI can prepare, compile and flag uncertainty. The architect validates the consequences." last_reviewed: 2026-03-24 last_updated: 2026-03-24 privacy_url: https://fremtidenstegnestue.dk/en/privacy related_pages: - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/bebyggelsesprocent relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/byggetilladelse relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/lokalplan relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge/concepts/br18 relation: related-concept - url: https://fremtidenstegnestue.dk/en/knowledge relation: hub status: published --- # Extension Cost — What Does It Cost to Build an Extension? Canonical URL: https://fremtidenstegnestue.dk/en/knowledge/articles/extension-cost Markdown URL: https://fremtidenstegnestue.dk/en/knowledge/articles/extension-cost.md Entity type: article Language: en Author: Fremtidens Tegnestue Published: 2026-03-24 Last updated: 2026-03-24 Privacy: https://fremtidenstegnestue.dk/en/privacy ## Short answer What does an extension to your house cost? See prices per m² for garden rooms, ground floor extensions and first floor additions — and what drives the price. ## Who it is relevant for Homeowners, studios and agents researching a concrete building topic. ## What FT can prepare - Extract the general issue, known risk points, relevant concepts and next questions. ## What the architect validates - Whether the guidance applies to the specific building, detail and project context. ## Article body You have outgrown the house. The children need their own bedrooms, the home office is a corner of the bedroom, and the kitchen feels too small every time you have guests. The neighbourhood is right, the primary school is five minutes away, and the house is paid for. But it is too small. What does it cost? ## Prices per m² — three types of extension **Garden room / conservatory: 8,000–18,000 DKK/m²** The lightest form. Typically an insulated structure with large glazed sections, connected to the existing house. Good for extra living space, but can feel cold in winter and warm in summer if not properly insulated. A garden room of 20 m² costs approximately 160,000–360,000 DKK. **Ground floor extension: 15,000–25,000 DKK/m²** A proper expansion of the house — brick, timber or lightweight block, with a foundation, roof and insulation matching the rest of the home. The most common choice. An extension of 30 m² typically costs 450,000–750,000 DKK. **First floor addition: 20,000–30,000 DKK/m²** When the plot is too small to extend outward, you can build upward. This requires the foundation and load-bearing walls to support additional load — a structural engineer must assess this. The price is higher because the construction is more complex. ## What is included in the price? A 30 m² ground floor extension typically breaks down as follows: - **Foundation:** 30,000–60,000 DKK — reinforced concrete or screw pile - **Shell (walls, roof):** 150,000–300,000 DKK — materials and masonry - **Insulation:** 15,000–40,000 DKK — walls, floor and roof to current standards - **Windows and doors:** 30,000–80,000 DKK — depending on size and quality - **Electrical and plumbing:** 20,000–50,000 DKK — sockets, lighting, possible heating and water - **Interior finish:** 30,000–60,000 DKK — floor, walls, ceiling, paint - **Connection to existing house:** 15,000–40,000 DKK — opening in exterior wall, adjustment work ## What drives the price up? **The foundation.** If the ground is difficult (soft substrate, high groundwater), the foundation costs significantly more. A geotechnical investigation (2,000–5,000 DKK) can save you from expensive surprises. **Connection to the existing house.** The more integrated the extension with the existing house, the more expensive. An opening in a load-bearing exterior wall requires a steel beam and structural engineer's calculations. **Material choice.** [Clay brick](/en/knowledge/materials/tegl) is more expensive than timber, but gives an appearance that matches most Danish houses. Timber is cheaper and faster, but requires maintenance. Rendered walls fall in between. **Roof form.** A flat roof is simpler and cheaper than a pitched roof that matches the existing house. But the [local plan](/en/knowledge/concepts/lokalplan) may require the extension to harmonise with the existing roof form. ## What do the rules say? Before you build, clarify: **[Plot coverage ratio](/en/knowledge/concepts/bebyggelsesprocent)** — typically 30% for detached houses. Everything you build counts: house, garage, carport, garden room, covered patio. Check what you already have, and what building rights remain. **[Local plan](/en/knowledge/concepts/lokalplan)** — may set requirements for materials, colours, roof form, height and distance to boundaries. Some local plans prohibit extensions in certain directions. **[Building Regulations (BR18)](/en/knowledge/concepts/br18)** — sets requirements for insulation, [energy rating](/en/knowledge/concepts/energimaerke), daylight, accessibility and structural safety. An extension must meet current energy requirements, even on a 1960s house. **[Building permit](/en/knowledge/concepts/byggetilladelse)** — must be applied for with the municipality. Processing time varies from 4 to 12 weeks. Start with a pre-application dialogue — it is free and gives you answers before you spend money on drawings. ## Typical extensions — and what they cost - **Kitchen/living room expansion by 15 m²:** 250,000–400,000 DKK - **Extra bedroom of 12 m²:** 200,000–350,000 DKK - **Garden room of 20 m²:** 160,000–360,000 DKK - **First floor of 40 m²:** 800,000–1,200,000 DKK - **Extension to brick villa of 25 m²:** 450,000–650,000 DKK ## Timeline An extension typically takes 3–6 months from building permit to moving in: - Design and application: 4–8 weeks - Building permit: 4–12 weeks - Construction itself: 8–16 weeks ## What is the right choice for your house? Not all houses suit all types of extension. A [brick villa](/en/knowledge/articles/brick-villa-renovation) has different possibilities from a [70s type house](/en/knowledge/articles/70s-type-house). The plot, coverage ratio, the style of the house and your budget determine what makes sense. Start by finding out what you actually lack — space, light, connection between rooms? The answer is not always more square metres. Sometimes a [new floor plan](/en/knowledge/articles/too-little-space) solves the problem more cheaply than an extension. ## Mistakes that cost dearly The most common mistakes with extensions rarely involve materials — they involve planning: - **Missing geotechnical survey.** Soft substrate or [high groundwater](/en/knowledge/articles/groundwater-under-the-house) can double the cost of the foundation. A soil test for 2,000–5,000 DKK can save 50,000–100,000 DKK in unexpected extra costs. - **Overlooked plot coverage ratio.** Always check what you have already built on the plot — garage, carport and covered patios count. Discovering it too late means money spent on drawings is wasted. - **Poor connection to the existing house.** Openings in exterior walls, roof junctions and transitions between old and new are where mistakes typically show up as [moisture](/en/knowledge/concepts/fugt) and cold bridges. This is precisely where an architect ensures the new connects properly with the old. - **Unrealistic budget.** Always add 10–15% for unforeseen costs. Especially in older houses, surprises emerge when things are first opened up. ## Extensions for different house types Not all houses suit the same type of extension. A [70s type house](/en/knowledge/articles/70s-type-house) often has flat roofs and lightweight construction that makes it easier to extend — but insulation standards require attention. A [brick villa](/en/knowledge/articles/brick-villa-renovation) has heavy walls and high quality, but planning regulations may limit possibilities if the house is in a conservation area. And if the family has [outgrown the house](/en/knowledge/articles/outgrown-the-house), it is worth considering whether extension or a completely new home makes more sense in the long run. *Sources: BR18, V&S Prisdata, [Bolius extension guide](https://bolius.dk).* [Start your dream here →](https://dialog.fremtidenstegnestue.dk/en) ## FAQ ### What does an extension cost per m²? A ground floor extension typically costs 15,000–25,000 DKK/m². A garden room 8,000–18,000 DKK/m². A first floor addition from 20,000 DKK/m². Prices vary with materials, complexity and region. ### Does an extension require a building permit? Yes, almost always. Extensions require a building permit from the municipality, and must comply with the plot coverage ratio, local plan, setback requirements and the Building Regulations' requirements for insulation and energy. ### How many m² am I allowed to build? It depends on the plot coverage ratio in the local plan — typically 30% for detached houses. On an 800 m² plot with 30% coverage, the total floor area may not exceed 240 m². ## Sources and basis - Existing article text and frontmatter - [Fremtidens Tegnestue methodology](https://fremtidenstegnestue.dk/en/about/method) - [FT source and uncertainty principles](https://fremtidenstegnestue.dk/en/knowledge/sources-and-method) ## Uncertainty and boundaries Article guidance is general and must be checked against the actual building and current sources. ## Next step Use related concepts or contact FT for a concrete case. ## Related pages - [Site Coverage Ratio in Denmark](https://fremtidenstegnestue.dk/en/knowledge/concepts/bebyggelsesprocent): related-concept - [Building Permits in Denmark](https://fremtidenstegnestue.dk/en/knowledge/concepts/byggetilladelse): related-concept - [Local Development Plans in Denmark](https://fremtidenstegnestue.dk/en/knowledge/concepts/lokalplan): related-concept - [BR18 — Danish Building Regulations 2018](https://fremtidenstegnestue.dk/en/knowledge/concepts/br18): related-concept - [Knowledge](https://fremtidenstegnestue.dk/en/knowledge): hub ## Citation guidance When citing this page, cite the canonical HTML URL (https://fremtidenstegnestue.dk/en/knowledge/articles/extension-cost) as the public source and use this Markdown URL only as the agent-readable representation. Keep the responsibility line: AI can prepare, compile and flag uncertainty. The architect validates the consequences.