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5 unexpected costs of extending your home

Make sure your budget by getting to grips with all the hidden costs of extending than many homeowners fail to consider.

4 min read

When taking on a big project like a rear extension, the last thing you want is to have your budget surprised by an unexpected cost. Yet with so much confusing information out there, and so many complex processes, it can be hard to properly work out what you have in store.

At Resi, we’ve helped thousands of homeowners create a happy space through their projects, and so know exactly what many first time improvers need a heads up on. To help you stay ahead of the curve, here are five unexpected costs to prepare for...


Before any tangible building can begin, you first need to get your groundworks in order. This refers to not only the foundations of your build, but also tree removal, drainage, and even the paths and patios attached to your extension.

Typically, your main builder will bring in a groundworker as a subcontractor, to complete these vital early stages. You can expect to pay between £90-£250 per day for their services, depending on their level of experience, and your location in the UK.

Groundworks usually take 2 weeks to complete the first stages, but may take longer if your project is complex or quite large. A groundworker might also be brought in towards the end to create pathways, adding in yet more costs to your construction budget.

Learn more about groundworks here.

Application fees

You’ll be happy to hear that this fee isn’t nearly as daunting as some of the others on here. If you’re applying for planning permission or a lawful development certificate on a single home, you can expect to pay between £180-£250. These prices vary on the local authority you’re dealing with.

While architects, such as Resi, can manage your application on your behalf, acting as your planning agent, this fee must be paid by yourself. At Resi, we’ll alert you to when this payment needs to be made.

Party wall agreements

The cost of your party wall can go either way, depending on your neighbours.

If during your proposed design phase you find out that your project will affect a neighbour’s wall, outbuilding, or boundary, you’ll need to serve a Party Wall Notice. Surveyors and other companies will generally charge between £65-£100 to arrange a notice to be served on your behalf. Alternatively, you can draft your own using examples set out in the Party Wall booklet. Or, if you’re using Resi, we’ll manage this on your behalf.

If within 14 days your neighbour gives written consent, your project can go ahead with no additional costs involved. However, if contest isn’t received, you’ll need to arrange a Party Wall Agreement. This is done through a party wall surveyor. In some cases, this professional can represent both you and your neighbour, however, your neighbour might appoint their own, or ask for a separate professional. All costs incurred by party wall surveyors must be covered by yourself.

A party wall surveyor can be anyone with decent experience of the processes involved, so costs can vary depending on who you choose. They could either be a professional surveyor, your architect, an engineer, or solicitor.

As a rough guide, you can expect to pay between £80 - £300 per hour for each surveyor involved. This can quickly add up, so make sure you have a couple of grand kept aside, in case this becomes an issue.

Learn more about party wall agreements.

Building regulations

They’re a set of statutory requirements that all buildings must meet, in order to be both legal and safe for habitation. They cover everything from structural engineering, thermal performance, sound proofing, all the way to fire safety.

Because of all the technical work involved, and the amount of professionals required, this is very expensive part of the process. You’ll need to set a significant part of your budget aside in order to cover each element. Here’s a breakdown of what to expect…

  • Application fees £650 - £1550
  • Architect £700 - £1200
  • Structural engineer £1200 - £3000
  • CCTV Surveyor £250 - £500
  • Party wall surveyor, see above

These prices can vary widely, and are most commonly affected by the size and complexity of your project. You’ll also find professional charging more in the South East of the UK.

Learn more about building regulations.

Project management

A project manager is there to organise the creation of your build, making sure your project is completed on time, on budget, and has a quality finish. They’ll over contractors and/or subcontractors, and deal with building controls, health and safety, and the utility companies. On top of this, they’ll also be coordinating any other professionals involved, such as your structural engineer. Basically, they wear a lot of organising hats.

Project managers can cost between 15-25% of your construction budget. The smaller the project, the higher the percentage. So if you want to save yourself some cash, you might want to consider taking on the role yourself.

Learn more about project management.

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