Extension Building Costs

A brief consideration of how much it will cost to build a home extension and how you can work with your architectural designer to achieve best value for money. This now includes prices for building eco house extensions, conversions, possibly doubling the space in a bungalow by replacing the roof, and 'whole life building costs'..


So if moving house is so expensive, what does it cost to build a home extension ?.

Here you will find the tools to help you work out outline budget prices for your extension project, with access to FREE estimates when you have decided exactly what you want and when.

Are Extension Building Costs Expensive ?

As with many such questions, the answer is, ‘it depends’.

First, of course it depends on the specification that you choose. It is pretty obvious that, for example, gold plated taps will cost more than white plastic ones. The same applies to the cost of insulation, heating, windows, and so on. Understanding the background to the costs of building your extension is key to getting best value for money.

House Extension Costs

It may not be so obvious, for example, that the type of wall you choose will also affect the cost. As will the shape of the building, the number of corners, the type of foundations required, and what style and construction of roof you would like, as well as the overall appearance that you – or your local ;planning authority – want to achieve.

Prices for Extending Buildings

Given the vast number of alternative material and design options, the price for any building extension will be greatly influenced by the architectural, as well as the structural approach taken by whoever designs your extension.

Ground floor extension estimate will also have a different set of considerations from upper floor extension quotations. Likewise pricing for new roof extensions will be different from extra space conversion quotations.

Price –v- Cost

Additionally, another factor is increasingly coming into the equation – ‘Life Time Costing’. Until quite recently many people have ignored this, often going for just the lowest priced building method.

Now, in response to Government imposed higher standards of construction via the building regulations to address their ‘climate change’ agenda, all buildings need to consider ‘life cycle costing’, in accordance with the general guidelines provided for ‘Whole Life Costing’.

As this states “…awarding contracts on the basis of lowest price tendered for construction works is rarely value for money; long-term value over the life of the asset is a much more reliable indicator”

A simple example might be using a roof covering that needs repairing, or even replacing, every 10 years or so, as against one that could last a hundred or more years. The first might cost less initially, but over the long term life of the building the added costs of repair / replacement could add up to several times more than this initial cost.

Costs and Energy Efficiency

A major emphasis in the October 2010 Building Regulations is the energy efficiency of buildings. This includes consideration of thermal efficiency, which includes thermal insulation, airtightness, and the use of on-site generated energy. Such energy embraces everything from grounds and air-source heat pumps, though bio-mass fuels, to wind and solar power, for example in producing hot water or electricity.

The “Building Envelope”
Of these cost and energy efficiency factors, the immediate and most important consideration is the types of ground floor, external walls, and roof that will be used in a building. In other words how the ‘Envelope’ of the structure will be constructed.

Each of these needs to meet minimum prescribed insulation ‘U-values’ and the building overall has to achieve both a minimum target level for air tightness and specified overall performance standards. These are determined using a Government “Standard Assessment Procedure” (SAP) method.

Wall Construction - Make-Up & Cost

Although predating the 2010 Building Regulations, an interesting introduction to this subject is given by a Home Building & Renovation article, What Will it Cost? - Constructing Walls. The 2010 Building Regs, of course, now call for 25% improvement over the earlier energy efficiency requirements, and this calls for much higher standards of insulation.

Considering Wall Thickness

This means there are now several things to bear in mind when considering the cost of external walls – weather protection, decoration and maintenance, energy efficiency, and thickness. With extensions, additions and other remodelling, renovation, or reconstruction works, wall thickness may be a highly important factor, possibly controlled by the thickness of present walls, proximity to boundaries, and the need to obtain greatest space inside the building by using the thinnest walls possible. An interesting consideration by the Energy Saving Trust can be found at Solid Wall Insulation.

The thickness of any external wall - brick, block, rammed earth, stone, straw bale, timber frame, or whatever – will be determined by the way in which it is insulated and the material used for insulation.

  • Old style ‘wet’ masonry walls, e.g. brick and block, now have to be much thicker than they used to be in order to accommodate extra insulation. For example, a brick inner and outer skin would require at least 60mm of PIR (rigid foam) insulation to achieve a U-vale of 0.25, meaning a wall thickness of 310mm.

    However, appearances can be deceptive and many ‘brick’ walls are now built using an inner timber frame leaf to replace block-work. This enables a greater thickness of insulation to be incorporated in the ‘cells’ between the timbers used for framing.

  • Straw bale walls are, by definition, at least the thickness of a straw bale, i.e. around 450 – 500mm minimum. While they have sometimes been just stacked one on top of another, to overcome potential instability and other problems, in the UK they are now generally held inside a timber surround frame. The following articles and sites may be of interest to those thinking of this material. Straw Bale Construction by Bruce King P.E, Civil Engineer, The Pros and Cons of Straw Bale Wall Construction in Green Building Ted Owens, Building With Awareness Blog, and a dissertation submitted at the Welsh School of Architecture.in 2007 Is Straw Bale Construction suitable for Self-Builders in Britain?

  • I-Beam or Twin Leaf walls are timber frame walls, usually associated with cellulose, e.g. Warmcell, insulation and are usually between 200mm and 300mm thick just for the timber frame element. These still require a ventilation cavity and external ‘rain screen’ cladding, e.g. 112.5mm brick (wet), or (dry clad) render, timber, tile/slate hanging, or brick/stone slips. With ‘dry clad’ walls the cavity requirement can also be reduced to circa 25mm.

  • Timber Frame wall construction can take many forms, from classical medieval frame buildings, though modern Green Oak reproductions and glulam or similar post and beam structures, to platform frame panel houses, SIP’s (Structural Insulated Panels) and composites, like timber frame with Hemcrete. Although pre-dating the 2010 Building Regulations, the UK Land Directory has published an interesting Self Build Guide – Timber Frame House

As will be seen, today, choosing the structure for an external wall depends on its thickness, which in turn depends on the insulation value required.

Any of these methods, as well as other forms of construction, can be used for extension projects.

Insulation Costs

There are many different types of insulation – rock wool, glass fibre, sheep wool, wood wool/fibre, rigid foam, cellulose (recycled), and more. Most of them are available under several different brands, and of course all have different performance characteristics and pricing structures.

SAP’s, EPC’s and Insulation
One difficulty encountered when trying to decide upon what type of insulation to choose is that the Building Regulations 2010 require both SAP’s (Standard Performance Assessments) and EPC’s (Energy Performance Certificates) which have to be both pre- and post- assessed before a building can be occupied. These allow freedom to decide on the mix of floor, wall, window, door and roof energy performance / insulation value. In turn these often depend upon the orientation, exposure, location and surroundings of buildings.

It is not therefore possible to give any hard and fast guidelines or cost indications of insulation, since these are both a personal preference and design driven.

Although pre-dating the 2010 Building Regulations, Robin Lancashire of TRADA (Timber Research & Development Association) published a most helpful article How to Pass the Airtightness Test in Spring 2007.

Of course, the methods and standards of airtightness to be achieved vary with both the types of external ‘building envelope’ to be employed and the method and provisions for achieving the desired level. Accordingly, once again it is not ;possible to give predetermined costs for this essential element.

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