Financial Planning: How much will my house extension cost?

A house extension can be one of the best investments that you will ever make. It can invariably add far more value in equity than it will cost to build. However the budget must be appropriate to the project and sensibly set against the client’s aspirations. This article discusses the complexities of cost control in bespoke residential work and what clients need to consider for a successful project.

Battersea House

First of all, in truth, due to the complexity and variability of these bespoke projects there is no accurate formula that you can use to ascertain construction costs. Every project is different and it is impossible to accurately estimate a project’s cost at its inception as there has been no design work completed. We use our own projects in our initial discussions with clients to build a picture of what the costs might be. This has been briefly analysed below with a review of four recently completed projects.

Hackney Extension

A refurbishment and semi-basement garden extension to a lower-ground floor flat.
Internal refurbishment = £1000/m2
Garden extension (semi-basement construction) = £3400/m2

Pod House

A part two storey full width rear extension, internal loft conversion and refurbishment throughout.
Internal refurbishment = £1100/m2
Rear extension = £2500/m2

Battersea House

A loft conversion with full width dormer, rear extension and full house refurbishment
Internal refurbishment = £1500/m2
Extension and loft = £2500/m2

Folded House

An innovative angular rear and side in-fill extension and house refurbishment.
Internal refurbishment = £1200/m2
Extension (piled foundations) = £2900/m2.

Average London Extension and Refurbishment Costs

In addition to the above we have completed a further four projects so far this year. Combined with these projects the spreadsheet below gives a reasonable idea about how you might set a budget based on a square meter area. Clearly there has to be ranges as each project’s scope and quality of fit-out varies:

  • Will all of the windows need replacing?
  • How many bathrooms are included?
  • How much do you want to spend on a kitchen?
  • How much custom joinery will there be?
  • What is the budget that you have available?
  • What is the main driving force behind the project?

These are just some of the questions that need to be considered along side ‘how big might your extension be?’

The above should assist with setting a starting budget. It is also useful to understand how costs are proportioned through a refurbishment project. Clearly the part with the most structural work is more expensive. About half of the cost of a single storey extension is in the foundations and structure. If this gets more complicated with basement construction or special footings (due to proximity to trees for example) then the costs increase. Kitchens and bathrooms are expensive rooms due to the extent of high quality fixtures and finishings required. Alternatively other rooms might just need some light decoration.

Project Costs vs Build Costs

In addition to the build costs there are further costs to consider. VAT is 20% and is chargeable on all domestic refurbishment projects. You should also consider a contingency on top of your build costs. Including these items will equate to about 80% of the overall project costs. Professional fees, planning and building control charges, Principal Designer (Health & Safety) duties, Party Wall processes will make up the rest on a typical build.

Clients will also have to consider whether they can stay in the property for the duration of the works. It may be worth considering phasing a project to ensure this can happen, this is usually more cost effective than moving out.


Accurate Cost Estimates – Using a Quantity Surveyor

We like to use Quantity Surveyors for all projects above £150,000, particularly if a client has little flexibility with their budget. In most cases a QS will save more money than they charge for their service. We typically engage a QS after the planning submission has been made. There are significant benefits of having accurate cost input at this time. The cost estimate is used to extrapolate budgets for the important fit-out elements such as the kitchen, glass work (doors/roof lights) or key floor finishes. These are often the design elements that have the maximum impact for a client and therefore it is imperative that these are right.


Design-led Fixed Price Construction

We are architects and designers first and foremost, quality of design and construction is imperative. Whilst this is an easy sentiment to make it is our design and procurement process which ensures this is the case. We are not affiliated with any contractors but know of many excellent building teams who we like to work with. Our preferred method of procuring a project is through a Fixed Price bid process with a full set of Technical Design drawings. This ensures you get an accurate market price for a clearly articulated design. This will remove potential conflict on site, ensures cost certainty and is how good builders prefer to operate.


Tailored Design and Added Value

Many of our clients are embarking on their first major renovation project with us. Whilst these projects require significant funding to execute it is important to focus on the key objectives and value added.

  • Your home can be tailored to your family’s requirements.
  • Kitchen spaces can be made more social, suitable for modern life, with better access to the garden.
  • Specific spaces, such as home working, children’s playrooms, or fitness studios, will improve the quality of your everyday life.
  • The additional equity you will create will more than offset the cost.
  • Your principal residence is exempt from Capital Gains Tax, profit is therefore untaxed.

If you have a project that you would like to discuss with us we would be more than happy to explain the timeframes, processes and costs in more detail with you. Please do not hesitate to get in touch. You can view more of our work here.