How Much Does a House Extension Cost?
Updated: Apr 2
To get an idea of how much an extension costs to build let us take a look at the process and costs for building a standard single storey house extension 3metres by 7metres.
ARCHITECTURAL DESIGN COSTS
A kitchen extension will have a huge impact on any home as it not only gives you more floor area but the opportunity to create an extremely desirable living space for all the family.
At the moment you are allowed to extend 3mt from the rear wall of the house within permitted development so for the sake of argument this extension project will be 3mt deep by 7mt wide.
You will need Design Plans, Building Plans and maybe a Planning Application £2,000 / £2,500
GENERAL SPECIFICATION & SCHEDULE COSTS
As an optional extra many designers will provide a detailed specification and schedule of works for the proposed build. The specification and schedule is mostly used for tendering, getting prices in from the builders, and for guiding the builders through the build process. £250 / £350
PARTY WALL AGREEMENT COSTS
A full width extension on a terraced property will take you right up to the boundaries so you will need to set up a Party Wall Agreement with two of your neighbours. £0 / £1,500
STRUCTURAL ENGINEER CALCULATIONS COSTS
As with most kitchen extensions you will most certainly be opening up the rear load-bearing wall of the house to create an open-plan kitchen and family room. This will involve a substantial chunk of steel to be installed for which you will require structural surveyors calculations and detailed drawings.
£500 / £600
BUILDING CONTROL FEES
The MAN always has to be paid so pay your Building Control Fees when you submit a commencement notice.
£750 / £1,000
STRIPPING OUT COSTS
There is always a certain amount of stripping out before you can start work. Get rid of old kitchen units, worktops, boiler, radiators, pipework, floor coverings as necessary.
£500 / £750
Generally, you have to excavate the entire floor area of the new extension area, about 450mm below ground level and 1000mm for trench foundations for all new walls. Approx 21m2 of floor area. All in about 16m3 of soil to be removed which is about 4 @ 4mt skips. £3,000 / £3,500
For a small project like this we tend to mix our own concrete. Make sure you get Building Control approval before you pour the foundations. We calculate you will need about 6m3 of concrete. £3000 / £3,500
DPC WORK COSTS
Build on top of foundations using trench blocks or semi-engineering bricks up to damp proof level. Install a decent dpc polythene membrane or damp proof course and ensure it overlaps with existing dpc on main building walls. £400 / £500
EXTERNAL WALLS COST
Nowadays, most of these type of extensions are built using block to the new boundary wall and salvaged old stock bricks to the end wall so that it matches up with the the existing building. Build all walls up to 2.4mt roof level. Because we are installing 4 metres of floor to ceiling folding patio doors we only for 9mt of wall in total. About 224 full width insulated blocks. About 22m2 of block work.
£3,500 / £4,500
ROOFING & ROOF GLASS OR WINDOWS COST
Here you have to decide the type of roof you like. For example, a pitched roof or a flat roof. You will also have to decide on the roof windows as you will need plenty of light for the enlarged kitchen family room.
Most modern kitchen extension tend to go for a flat roof. We have opted for a flat roof with roof lantern.
£6,000 / £6,500
STRUCTURAL OPENINGS / ALTERATIONS COST
Opening up the main house wall is serious work and a substantial chunk of steel has to be installed as per the structural surveyors report.
£1,700 / £2,500
DRAINS / DRAINAGE / RAIN WATER COSTS
The soilstack for the upper floor bathrooms may have to be removed and a new soil stack installed that takes the waste of the upper floors. You will also have to relocate and reroute rain water pipes.
Existing gullies will have to be capped or rerouted as necessary to take rainwater from new extension roof and for waste water from the new kitchen. If you happen to live in the countryside you may have to install a new soakaway.
£2,000 / £2,500
CONCRETE FLOOR COSTS
A completely new concrete floor has to be laid that covers the entire extension. This new floor will generally be insulated and depending on the choices you make, may have underfloor heating installed that connects up to the existing heating system. You are likely to require 5m3 of concrete for an average floor of this size.
£3,000 / £3,300
INTERIOR WALLS / PARTITIONS COST
If you are remodeling the old rear reception and kitchen then you may be installing a utility room or larder that requires a bit of stud partition or block wall.
£500 / £750
EXTERIOR WINDOWS & DOORS COST
In most cases all you need is about 4mt of folding, sliding, or bi-fold doors to the garden to create that modern look.
£4,000 / £4,500
ELECTRICAL & LIGHTING COSTS
Generally, I recommend that you put in a new ring main circuit for the new extension with up to about 8 twin sockets. This covers you for worktop points, appliances and wall sockets. You will also need a 6mm circuit for the oven and hob. I recommend that you install a new circuit at this point.
Low voltage lights also work really well so I would allow for about 12 low voltage recessed lights to be installed. You might also want to install a security light to the garden.
£1,200 / £1,750
PLASTERING & RENDERING COSTS
External walls will require rendering and the internal side of the new walls will need to be covered with insulated plasterboard. Board over ceiling, don’t forget the vapour barrier, patch upas necessary and skim allold and new walls to the new extension and old kitchen dining area.
£1,200 / £1,800
You should always allow a bit for joinery as you may have a new door and frame to the kitchen and utility room. Don’t forget locks, handles, timber flooring repairs and any skirting that might be required.
£300 / £500
PLUMBING & GAS COSTS
Plumbing means re-routing hot and cold water supplies as required for the sink, appliances and outside tap. You also have to install new waste pipe as necessary. If you have a gas hob then a gas supply will also be required for the hob and possibly a new or relocated boiler.
£400 / £600
BOILER & HEATING COSTS
The boiler for the property is generally situated in the kitchen in these type of homes which means it has to be relocated or a new one installed. If the boiler is more than 7 or 8 years old then it probably makes sense to install a new one rather than simply relocate the old one.
£2,000 / £2,500
KITCHEN UNITS WORKTOPS & APPLIANCES COST
This is big ticket item and prices range from £2/3k for an IKEA job to a high priced designer package for £25/£30k. The choice is yours but for this example we will allow a low-to-average price for a new fitted kitchen. £6,000 / £8,000
TILING / SPLASHBACKS COST
A small job but it mustn’t be overlooked as some of these tiles can cost an arm or a leg. £300 / £500
FLOORING FINISH COSTS
Depending on your taste the popular choices are wood and tile with prices ranging from £8/£100 per m2 . For our example you will need about 45m2 of tiles, which includes the old kitchen diner room, and we have allowed £50 per m2.
£3,000 / £3,500
As for the interior painting is it more or less 2 large room plus a potential utility room and possibly a bit more work in the hallway to square off. £850 / £1,200
By exterior painting we suggest you repaint anything that has to be painted such as the windows to the upper floors, facia boards, soffits and pipes plus new rendering.
£850 / £1,250
PATIO & FENCING COSTS
In most cases there will be a little bit of fencing reinstatement works to be done. As for the patio, this can be as elaborate or as simple as you desire. Generally, a full width patio about 3mt deep will work well. That means you will require patio slabs to cover an area of 15m2 .
£2,500 / £3,000
LANDSCAPE SOFT COSTS
Soft landscaping refers to lawn, plants and shrubs. We have allowed enough to get some colour into your garden.
£1,250 / £1,700
The word Preliminaries relate to preparatory measures to enable the work to take place and the cost of keeping the job going. You can include expenditure such as:
Waste Removal / Skip Hire / £1500
Plant Hire / Special Equipment / £0
Temporary Power / Water / £0
Toilets / Sheds / Offices / Stores £0
Security Access / Grills / Safety Rails / Hoarding / Dust Sheets / £100
Scaffolding / Cranes / Lifts / Supports / Hoists / £250
Insurance / Permits / Parking / £500
Running Costs / Power / Water / £0
Protective Clothes / £0
Cleaner / £0
Keys Cut / £0
Total Prelim Costs £2,350 / £2,500
So as you can see the costs mount up and can go up or down depending on the various choices you make. One tip I will give you is to choose a designer that knows how to add value to your home and that way you will be on a winning ticket right from the start.
After 40 years working the London property market, Mitehart Design & Build know a bit about extensions, in fact you could call us experts. If you need advice or a Free Quote for any type of property extension then let your fingers do the walking...
020 8676 0773 / 07930 837 813