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A guide to working out the total cost of your project

From architectural fees to your construction costs, delve into this complete guide to working out the cost of your extension, conversion, or refurbishment.

6 min read

How much is your project going to cost? It’s likely going to be the big question hanging in the air and one a lot of homeowners struggle to answer. After all, with all the different stages, professionals, and fees, it can be easy to get lost in the numbers.

However, whether you're funding your project through savings or through a loan, the last thing you want is to have your funds fall short. That’s why, at Resi, our Finance team work with our customers to nail down their budgets early on, so there’s no nasty surprises on the horizon.

How do we work out project costs? Here’s a breakdown of our calculations...

Construction Estimate

The biggest chunk of your budget will be going towards construction. This will include materials, labour day-rates, plus the miscellaneous on-site costs, such as hiring skips or portaloos.

You’ll typically find that builders price their services on the size of the project, whether they’re building or refurbishing, and the standard of work required.

For example, for a project located in the South of England, construction is likely to be priced as…

Basic build
£1500/m2 new build areas
£800/m2 refurbished areas

Standard, high-quality build
£1650/m2 new build areas
£875/m2 refurbished areas

Premium quality build
£1800/m2 new build areas
£950/m2 refurbished areas

Learn more about the price of construction.


Your next consideration will be VAT, which currently stands at 20%. Both your labour and materials will be subject to this rate, especially if you’re using a contractor.

However, there are some exceptions…

  • If you use local tradesmen, who aren’t VAT registered, you won’t have to pay 20% on their labour but will still have to pay VAT on materials.

  • Work to listed buildings is eligible for VAT relief, paying a zero rate.

  • Conversions to an existing dwelling which changes them into multiple units, pay a reduced rate of 5%.

  • Likewise, projects on buildings that have been unoccupied for at least two years will get the 5% rate.

In order to receive these relief benefits, you’ll need to use a VAT-registered contractor, as you won’t be able to reclaim the VAT yourself.


At Resi, we recommend you put 10% of what your construction costs aside. This will help protect you in the event something goes wrong and stop you from having to scramble for extra cash at the last minute.

While everyone hopes their project will go off without a hitch, you need to remember that construction alone can take months to complete, and there are so many factors outside of your control. Be this the economy, the weather, and even your suppliers.

As with everything, it’s better to be safe than sorry.

Kitchen refurbishment

Bathroom and Kitchens

Both bathrooms and kitchen require special attention when being fitted and come with their own unique costs.

If you’re looking to get a rough cost of these rooms ahead of time, a good rule of thumb is to base your calculations on the current value of your property. Bathrooms tend to cost 1%-4% of your home’s value and kitchens 2%-5%.

For example: a property that’s valued at £400,000 would probably pay between £4,000 - 16,000 for a bathroom and £8,000 - £20,000 for their kitchen.

These estimates can be broken up into these key components...


  • Installation (20%)
  • Cabinets (16%)
  • Fixtures (15%)
  • Faucets / plumbing (14%)
  • Flooring (9%)
  • Countertops (7%)
  • Walls / Ceilings (5%)
  • Design fees (4%)
  • Doors / windows (4%)
  • Other (1%)


  • Cabinents (30%)
  • Countertops (10%)
  • Appliances (15%)
  • Plumbing (5%)
  • Electrics (5%)
  • Flooring (5%)
  • Other (10%)
  • Labour, plus contingency (10%)


Any project, whether it’s a conversion, extension, or new build, is a big investment. And like all investments, you’ll want to make sure yours is protected. That’s where warranty comes in.

You can cover your project between 2, 6, and 10 years with each coming at a percentage cost of your build. We recommend getting as much protection as possible; a 10-year warranty policy will charge 2% of your construction costs.

Devonshire 43

Architectural fees

For a traditional architect, fees can be between 5% - 15% of construction costs, depending on whether or not you bring them in to project manage.

At Resi, we tailor our prices to your project and can guide you from step one, all the way to getting ready to build.

Our packages include…

Get a quick quote for your project here.

Building regulations

They cover everything from the structure, thermal performance, sound-proofing, drainage, all the way to fire safety - to name just a few!

To ensure your project is in line with UK building regulations, we recommend you commission a set of technical drawings of your proposed build, so your contractor has detailed instruction on meeting all legal requirements.

In order to put this package together, you’ll need to budget for several services.

  • Our building regulation package starts from £775 and is determined by the size and complexity of your project.
  • A structural engineer typically costs £950-2500
  • An approved inspector £900-2000
  • CCTV drainage survey £250-350

Learn more about building regulations.

Reading chair by window

Party wall matters

If you have neighbours, you’ll need to factor in getting their consent for your project. This will involve sending out a party wall notice and should they not give written consent to this request, you’ll need to put together a party wall agreement. You’ll have to get a party wall surveyor involved and potentially even a second, should your neighbour request one. All costs, including those of next door, will be on you to cover.

As a guide, we would recommend putting aside £600-700 per neighbouring household, though you might need more to cover any additional costs per hour.

Learn more about party wall matters.

Miscellaneous costs

Alongside these main costs, you should be prepared for the odd expense to your local council. Both for your planning application and for securing building control approval.

We also recommend taking out the proper insurance policies to cover yourself during the build. Again, this might require extra budgeting.

Financing your project with Resi…

By working with Resi, you’ll have access to our unique Finance service, which allows us to nail down your budget early on and help you to secure the best funding.

While many people look at short-term costs, don’t forget to consider the big picture. Unless you have a nice pot of savings, it’s likely you’ll be financing your new build through a loan. One of the best things you can do when choosing your provider is to go with a specialist lender, who has experience in handling property-related investments.

As official partners to the Mortgage Advice Bureau, our Finance service is able to source the best lenders and rates for your project, working with your designers to ensure you get the right amount of funding at the right time.

We’ll also help ensure your investment is properly protected by sorting out the best insurance policies for you. As the COVID-19 crisis illustrated, you want to make sure that no matter what happens in regards to your health or employment, you can still make your repayments.

Get a tailored project estimate with Resi Finance.

Please note:

  • You may have to pay an early repayment charge to your existing lender if you remortgage.
  • Your home may be repossessed if you do not keep up repayments on your mortgage.
  • There may be a fee for mortgage advice. The actual amount you pay will depend upon your circumstances. The fee is up to 1%, but a typical fee is £495.

See how much you can borrow

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Track record in getting planning approval


Projects undertaken across the UK

34 days

Average time from survey to planning