How much will my House Extension cost and how long will it take to finish?

House Extension Builders in SE2- Sanderstead

House Extensions can be difficult to get a clear understanding of all the costs, it is a good idea to get a proper extension cost before you commit to any one Builder, sometimes it is even worth using an extension cost calculator to give yourself a better price overview.

How much does an extension cost?

The cost of building a House Extension varies depending on what you require, maybe a Kitchen extension, two storey extension, single storey extension cost , Garage conversion all of these will differ in price.

Where you live in London also plays an important factor too, such as Sanderstead CR2 South London. Smiley Building services carry out House developments in the South East London areas, so have a wealth of experience in providing exact quotations. Below we give you some rough numbers for each kind of extension that can help give you a general idea.

Single storey extensions

In 2017, an estimate for the cost of a single storey extension in England was anywhere between £1,200 and £1,500 per m2. In London and the South East, this was anywhere between £1,500 and £2,000+ per m2. So let's say you're looking for a 5m x 5m Single Storey House Extension might be around £30,000 In SE London, maybe even up to £50,000+, remember that's just for building the extension. We now add on 10-15% in professional fees for the architect, planning application, building regulations, structural engineer plus the VAT. So a single storey extension could cost £39,000 at the lower end. £65,000 in SE London at the upper end. Check out costs for yourself by getting three quotes from builders local to you.

Two storey extensions

With a Two Storey extension, if both storeys are the same size, a good rule is to add 50% extra to the build cost of a single storey extension., So the £39,000 becomes£58,500 and add 10% for professional fees and VAT = £76,050 . Again, London and the South East will more likely be pricier.

Kitchen or bathroom extensions

Building Plans and permission need to account for plumbing to be installed with a professional plumbing team, for a bathroom, you'll need to add about £5,000.although this will depend on your finishes and what you actually need to install in the new bathroom (toilet, shower, bath, wet room).

A Kitchen will cost more than a bathroom, around £10,000 will get you a low/mid-range kitchen, and a top of the range will be as expensive as your tastes are.

Bungalow Extensions

Bungalow extensions are the same cost as a single storey house extension, as they are all on the same level.

Loft Conversions

Ceiling height is no longer part of Building Regulations, but there are still minimum heights for practical purposes, so take note of this when doing measurements. Most rooms today are built at a Floor to ceiling height of 2.4 Metres although heights of 2.1 Metres are acceptable. When you have sloping ceilings from the roof, a good rule is to have at least half of the room at a height of 2.4 Metres.

Finishings and fittings

With all types of House Extensions, the extras can sometimes add up and lead to a hefty bill. If you would be happy with painted walls, carpet or laminated wooden floors, and standard lighting then your costs will be on the lower side, however tiling, fitted wardrobes, bespoke flooring and upmarket fittings will all add to the final cost of build.

Windows Choices

Many people in their excitement to extend forget about windows, but they can be an expensive addition to a modest extension. Remember the bigger the window, the larger the cost due to the increased levels of glazing, bi-fold doors, and enhanced insulation.

Trees in the area

Trees can also cause complications as some trees are protected by Tree Preservation Orders, and will need to be taken into account when you are requesting planning permission. If you remove or alter a tree without the correct permissions, you could end up in big trouble.

Building site constraints

If you have a complex site, or have limited access, you may need to ask your builder to factor in any extra costs. Some examples may include Soil type that demands a specific building technique or material changes to an existing structure, any changes to steelwork or walls will also mean additional costs. Ease of access to the actual site if it proves difficult to manoeuvre tools and materials, not to mention if you need to move drainage, pipework, gas meters and so on this will require extra planning and cost

Other things to think about

You should get 1-3 quotes for any house extension to your home, but do not bother with any more than that. You will get a feel for what the market rate is, as well as the type of Builder you relate to and the ones you feel are most capable of completing your extension to your requirements. You should not confuse an estimate with a quote, as when builders or architects first visit a property they'll give you an estimate, which is only a starting point but it can differ greatly to an actual quote. Good advice is to have someone else with you when receiving your actual quote, this will normally be sent in writing after everyone has gone away and done some thorough calculations. This is your actual quotation and is what the project should cost and what you will pay.

Useful Links and damp

How much will my extension cost? is a bit like asking how long is a piece of string. Each project will be different and involve different dimensions and finishes. However, there is help at hand there are a number of extension cost calculators available on line. The following information is taken from Household , where you can view the full article.


Smiley Building Services are endorsed by “Trustmark”, who are the only UK Government Endorsed Quality building Scheme for work carried out around your home.

Several motivations fuel TrustMark's mission, chiefly to assist homeowners with implementing energy-saving upgrades in their dwellings and to bolster the level of confidence they possess while engaging in such undertakings. It is imperative to possess a well-defined vision of your intended home improvements, acquaint yourself with the potential expenses and expected duration of projects. Bear in mind that proficient professionals are typically sought after, hence there could be a necessity to be patient in order to secure the services of the ideal tradesperson for your task.


Smiley Building Services are a full member of Federation of Master Builders, who vet all members as reputable, and certify them as dependable local tradesmen.

The Federation of Master Builders (FMB) stands as the most prominent trade association for the UK's construction sector, dedicated to representing the interests of small to midsize construction businesses. Its mission is to serve as the definitive directory of reputable, certified, and dependable local craftspeople.

To be part of the FMB, individual contractors and construction enterprises of small to medium scale in the UK must have a business history of at least one year. The FMB isn't exclusive to SME builders alone; if your activities intersect with the SME builder industry and you uphold the FMB's commitment to excellence and industry standards, you may be eligible for FMB associateship. For further information, consider reviewing the criteria for becoming an Associate member.

The FMB separates itself from similar entities by requiring a membership that's not just handed out; it's awarded based on merit. Applicants are subject to a rigorous screening and an unbiased review of their work to ensure standards are met. This meticulous vetting is integral to providing clients with the assurance that they are engaging with a business that upholds quality and excellence in its services.