Extending your home can completely change the way you live. A well-designed extension can create a larger kitchen, an additional bedroom, a new home office or the open-plan family space you have always wanted.
One of the first questions most homeowners understandably ask is: how much does a house extension cost?
There is no single figure that applies to every project. Two extensions of exactly the same floor area can have very different final costs depending on their design, specification and structural requirements.
Understanding what influences the price can help you establish a realistic budget before work begins.
The Size of the Extension
Floor area is naturally one of the biggest factors affecting the overall cost.
A small single-storey rear extension will normally require fewer materials and less labour than a substantial wraparound or two-storey extension.
However, size should never be considered in isolation.
A relatively small extension containing extensive structural alterations, premium glazing and a high-specification kitchen could potentially cost more than a larger but simpler structure.
Single or Two Storeys?
A two-storey extension requires additional construction, but it can sometimes represent better value per square metre because certain costs are already required for the ground floor.
Foundations, site preparation and the roof are examples of elements that may make gaining additional first-floor accommodation particularly worthwhile.
The right solution will depend upon your home, your budget and what you want the completed extension to achieve.
Biggin Renovations provides a range of house extension services in Biggin Hill and surrounding areas.
Structural Alterations
Many modern extensions are designed to become part of the existing home rather than simply adding another separate room.
Creating a large kitchen-diner, for example, may involve removing an exterior wall or existing internal load-bearing walls.
This can require:
- Structural calculations
- Steel beams or RSJs
- Temporary structural support
- New foundations or padstones
- Building Regulations approval
Identifying these requirements during the planning stage is far better than discovering them after construction has started.
Kitchens and Bathrooms
The intended use of your extension can significantly influence the budget.
A simple additional living room may require relatively straightforward finishing work.
A kitchen extension could include cabinetry, worktops, appliances, plumbing, electrics, flooring and lighting.
Bathrooms introduce sanitaryware, waterproofing, drainage, plumbing and tiling.
Biggin Renovations carries out both extension construction and complete property renovations, allowing these different elements to be coordinated within one project.
Doors, Windows and Glazing
Large areas of glazing can create spectacular extensions filled with natural light.
Bi-fold doors, sliding doors, roof lanterns and large-format windows can all increase the specification and therefore the budget.
The important consideration is not simply choosing the biggest glazed area possible. Good design should balance natural light, privacy, energy efficiency, ventilation and how the space will actually be used.
Ground Conditions
Everything beneath the extension matters too.
Foundation requirements can vary depending on:
- Ground conditions
- Nearby trees
- Existing drainage
- The property’s construction
- The size and design of the extension
- Local site conditions
Groundworks are therefore an important part of any realistic extension quotation.
Planning and Professional Fees
Your construction budget may also need to account for professional services.
Depending on the project, these might include architectural drawings, structural engineering, planning applications, Building Regulations and other specialist reports.
Not every extension requires full planning permission because some projects can potentially proceed under permitted development rights. This should always be established for the individual property rather than assumed.
Specification Makes a Major Difference
One of the easiest ways for a renovation budget to grow is through specification changes.
Flooring costing £30 per square metre and flooring costing £100 per square metre perform the same basic function but have a very different impact across a large extension.
The same applies to kitchens, sanitaryware, lighting, glazing, doors and finishes.
Agreeing the desired specification early helps produce a much more meaningful quotation.
Include a Contingency
Even a carefully planned building project can uncover something unexpected.
Older properties in particular can reveal issues once walls, floors or ceilings are opened.
A sensible contingency provides flexibility without forcing important decisions to be made under financial pressure.
Look Beyond the Cheapest Quote
Choosing purely on price can be a false economy.
A detailed quotation should make it clear what is included, what specification has been allowed for and which elements may be additional.
Compare the scope of work rather than simply comparing the number at the bottom of the page.
Start With the Space You Actually Need
Before asking, “How big an extension can we build?”, a better question is often, “What do we need the house to do better?”
The answer may be more kitchen space, better natural light, another bedroom or improved connection between the house and garden.
Designing around those objectives can create a better result and sometimes prevents unnecessary construction.
Biggin Renovations Group provides renovation, extension, conversion and structural building services across South London, Surrey and Kent.
To discuss a proposed extension and arrange a free no-obligation consultation, call 07507 476 504.
Frequently Asked Questions
How much should I budget for a house extension?
There is no universal figure. Size, structural work, ground conditions, glazing, internal finishes and the intended use of the extension can all significantly affect cost.
Is a two-storey extension much more expensive?
It will cost more overall, but the additional first-floor space can sometimes offer good value because some costs are already required for the ground-floor construction.
Does a kitchen extension cost more than a normal extension?
It can, because cabinetry, appliances, plumbing, electrics and worktops are additional to the structural construction.
Should I include a contingency?
Yes. Keeping part of the budget available for unforeseen circumstances is sensible, particularly when working on older properties.
Can Biggin Renovations manage the whole project?
Biggin Renovations provides extension, renovation, conversion and associated structural services, allowing multiple elements of a project to be coordinated through one experienced team.
