Astrid Madsen
Planning to extend your home and wondering how much does an extension cost? Here are the main budget elements to be aware of.
Over the past few years, much research outlines the dangers of budgeting under an average cost per sqm/sqft basis. Nowhere does this become clearer than when we look to budget and cost house extensions.
Ballpark Extension Costings
How much does an extension cost?
The Royal Institute of the Architects of Ireland (RIAI) published a cost guide that contains average costs for extensions ranging from €1,900 to €3,400 per sqm. This range in itself outlines the difficulty in accurately budgeting based on an average cost.
The reality is that no extension is average and few are similar in design, layout, and use. But it is useful to know the main items that drive up extension costs.
Why Averages Are Misleading
House extension cost estimates can be deceiving. Averages may give you a rough idea, but they often fail to account for the unique aspects of your project. Extensions vary widely in design, size, and complexity. Therefore, it’s crucial to dive deeper into the specific budget elements that can impact your project.
Demolition and Waste Removal Costs
Demo, Waste Removal, and Services
Like any building project, the first issue to address is the demolition and removal of existing structures that stand in the way of your new extension.
Demolition Costs: Demolition costs can add significantly to the overall extension cost, especially for older properties that may contain hazardous substances like asbestos. The cost of demolition varies based on what needs to be removed and may involve hazardous materials removal.
Waste Removal: Waste removal is an additional expense, and accessibility can affect costs. In cases like terraced houses, waste removal may involve wheelbarrowing waste through the house, making it more time-consuming and costly.
Utilities: Extension projects often require altering or relocating existing foul and surface water installations and networks below the ground. This can include soakaways, rainwater retention systems, or alterations to utility services like gas, water, and electricity meters. Keep in mind that utility companies may require a capital contribution for alterations.
Structural Costs
Structural Support Extension Cost
Consider the structural implications when planning your extension. Open-plan designs often require removing existing walls, which can lead to the installation of large and costly steel beams. The cost of steel has increased in recent times and can range from €3,500/£3,000 to €4,000/£3,500 per tonne.
Additionally, uncovering existing structural issues during construction is common. Problems like inadequate foundations, crumbling walls, or poorly connected extensions can necessitate additional structural work and should be budgeted for.
Underpinning existing foundations is another potential expense, with costs ranging from €600/£500 to €1,500/£1,200 per linear meter, depending on the depth required.
Glazing Costs
Glazing Cost
Extensions often incorporate more natural light through fully glazed walls or rooflights. The choice of materials and size of windows and doors significantly impact costs.
Window Costs: Window costs vary based on materials and size. Prices can start at around €350/£300 per sqm for standard options and go over €800/£750 per sqm for premium choices.
Rooflights: The cost of rooflights depends on whether you opt for off-the-shelf or bespoke options. Standard flush rooflights are more affordable, starting at less than €/£1k per sqm, while lantern-type rooflights can cost €1,800-€2,000/£1,500 per sqm.
Consider lead times when planning, as waiting periods of several weeks for glazed products are common.
Roofing Costs
Roof Extension Cost
Roof construction for extensions can be straightforward in standalone structures. However, factors like roof type and finish can influence costs.
Roof Structure: Costs depend on the size and type of members and supports. Roof finish costs range from €75-80/£70 per sqm for tiles to €180-200/£180 per sqm for premium slate and metal finishes.
Existing roof alterations during extension work may lead to unexpected expenses, such as water damage repair or the addition of membranes or felts.
Keep in mind that selecting a flat roof could impact your house insurance premium based on the total roof area.
Internal Finishes
Internal wall, floor, and ceiling finishes are determined by your project’s layout. Extensions often involve upgrading insulation on existing structures, which may qualify for grants. Plastering, tiling, decoration, and internal joinery are influenced by material choices and quantity.
Floor Finishes: Flooring costs vary widely, from €10/£8 per sqm for laminate to over €150/£130 per sqm for decorative timber floors. Matching the extension with the existing property may require additional floor work.
Internal Joinery: Doors, architraves, skirting boards, and other internal joinery items should be budgeted for. Costs range from a few hundred euros for standard doors to several hundred euros for bespoke joinery.
Electricity and Systems Costs
Electric Cost for Extensions
Electrical works in extensions often require complete rewiring, impacting the entire house. The number of electrical sockets, lighting fixtures, and smart technology additions can lead to cost overruns.
Electrical Items: Many electrical items cost over €100/£80 per point, so planning detailed lighting layouts in advance can prevent budget issues.
External Lighting: Don’t forget about external lighting for practicality and security. Smart technology, sound systems, intercoms, and other amenities also come with a price tag.
Fit-Out Costs
Without a doubt, one of the most significant expenses in extensions is the selection of kitchen, utility, and built-in furniture.
Kitchen Costs: Kitchen costs vary based on design, layout, style, and finish. Prices can range from a few thousand euros for a basic kitchen to €30,000/£28,000 and above for a designer kitchen.
Countertops: Countertop materials like laminate, timber, granite, quartz, and engineered alternatives can range from €/£1k to over €/£15k.
Landscaping Costs
Extensions inevitably impact outdoor areas. You’ll need to make allowances for damage to your garden, including storage of materials and equipment, and potential disruption to drainage and services.
Patio or Paved Area: Budget for a patio or paved area alongside the property, typically costing around €150/£120 per sqm, including excavation, disposal, filling, and materials.
Garden Repairs: Consider repairs to grassed areas, kerbing, planning, raised plan
