Cost of building or extending a house

Unexpected Build Costs

Your home is often your biggest investment, your largest asset.  Building, extending or renovating your home is therefore a costly undertaking that will require good project management and budgeting. And you should manage your expectations – there will inevitably be unexpected costs when building a house.  It’s therefore always advisable to add in a contingency budget of at least 10% to cover these unforeseen costs.  Allowing up to 20% would be safer!

We’re often asked what unexpected, additional costs are likely to come up.  This is difficult to say until your works have started as it will be project specific.  However, from our experience renovating and building homes over the years, I’ve outlined below the most common areas where unexpected costs are likely to arise.

Bear in mind that unexpected costs when building a house may come in the form of labour, materials or services/utilities.  Or a mix of all of these.

Either way, before progressing with any additional, unexpected works, your builder should always communicate with you.  Explaining why additional works and costs are necessary, and managing your expectations around these, is all part of meaningful communication that should regularly take place between the customer and builder. 

Permits, Certificates and Sign Off

Bat Survey for planning application

Before you even start your build or extension, the Council or other may require permits or specific surveys to satisfy certain requirements.   For example, an ecology assessment to check for roosting bats or nesting birds.  A biodiversity survey to assess net gain, a new building requirement that comes in to play in November 2023.  Planning and building reg. requirements are constantly evolving, with national and local expectations in place.

Likewise, Building Control which is separate from the Planning Office, have requirements for sign off at every stage of your build which will require sectional drawings, electrical and gas certificates and other items specific to your project.

These all come with associated costs and may bring unexpected costs when building a house.

Ground works

Ground works when building a house

Where no unforeseen permits or certificates are required, additional ground works are often the starting point of additional unexpected costs when building a house.  For example, there may be a requirement to dig foundations deeper than expected on instruction from Building Control.  Alternatively unexpected soil conditions may have different requirements.  Remember that Building Control need to sign off every stage of your build, so if there is something they deem necessary, it needs to be implemented in order to get your Building Control certificate at the end of your build. 

Another possibility if you are extending your home, we may find out that your existing foundations are piled due to less stable ground conditions on your site.  This would then require additional works to build the foundations of your extension.

Where additional ground and preparation works are deemed necessary, additional costs will therefore be incurred for labour, materials, machinery and clearance.

Drainage & Pipework

When building or extending a home, the ground works and plumbing crews will need to link in to mains drainage and/or any existing house pipework.  The location of the drains and pipework, along with the age and condition of any existing pipework, will all have an impact on what is needed to ensure good drainage in your new or extended home.  Within a quote, a builder may make an allowance for moving drains or similar.  However, until ground works begin and there is visibility on the location and condition of the existing pipework, it will be difficult to estimate how much work and cost is involved to future-proof the drainage.  Replacing existing may not be included in your initial quote, but may be deemed necessary.  Any additional cost above expectations will be passed from the builder to the customer.

Some works may require that the service provider (e.g. your water or gas supplier) requires site visit to assess and / or carry out works directly – the service provider will likewise charge for their time and involvement.

Drain location when building a house

Unfortunately, drainage and pipework is never an attractive additional cost.  It’s not something you will visibly see at the end of your build project.  But it’s a necessary evil.  It’s crucial to get the drainage and pipework right at the outset to avoid future issues so it’s a worthwhile expense to swallow, albeit grudgingly.


Ordinarily a builder will allow for a specified number of grab lorries and skips in their quote.  Any additional skips or grab lorries then above quoted allowance will likely have a cost that is passed from builder to customer. 

However, don’t underestimate the cost savings to be incurred by finding alternative clearance methods.  For example, giving your old kitchen or conservatory away for free will save you time and money.  The “buyer” will need to come and dismantle it and take it away saving you labour and skip loads.  Sometimes it’s worth posting these on ebay for min bid of £1, and you may even get takers that are prepared to pay you for them.

For other cost saving tips on clearance, have a read of our blog Demolition Therapy.


Your electrician will be out to run through first fix electrics with you once the shell of your build or extension has gone up.   Ordinarily, unless specified otherwise, a builder will allow for a certain number of “points” (sockets, lights etc.) within your build quote.  This will also ordinarily be based on standard materials where they are being provided, such as wall plates and spotlights.

The builder will have pre-agreed a price per point with their electrician, which is then factored in to their quote to you where electrics are included.  If electrics are not included in your build price, but coming separately and directly from the electrician, an allowance will likewise be factored in by the electrician in their quote to you.

Therefore, should you decide to go above the allowance made, the cost for additional “points” will be passed on to you.  This often therefore becomes another of the unexpected costs when building a house.  Likewise, if you prefer specific sockets or specific lights, these will be at your own additional cost.

Personally, this isn’t an area I would ever recommend cutting back to save money.  Sockets and lights are a visible part of the interior finish of your home.  Thousands will have been spent on your foundations, insulation, sub floor etc., none of which is visible when your build is finished.  Although it’s undoubtedly reassuring to know it’s there.

So scrimping on interior finishes such as lights because your budget is running out at the end of a build project is not ideal.  These are the items that you will look at daily that will be testimony to the thousands you have spent on your home.  If you want a fancy light switch, go for it!  There’s an abundance of options out there now that really add to the aesthetic finish of your home.  It’s all in the final detail.

Also, from a resale perspective bringing value to your home, these details are noticed.

Dowsing & Reynolds Smoked Gold Switches
Switches from the Smoked Gold Range of Dowsing & Reynolds

Another area of electrics that may bring about unexpected costs when building a home is where you need to upgrade existing electrics.  If you are adding an extension to an existing house, it may be that you need to upgrade the electrics or wiring in the existing parts of your house for the whole system to be compatible and work effectively throughout.  This same principle applies to the plumbing and pipework too.

The builder is unlikely to have made an allowance in their quote for upgrading existing electrics and plumbing, unless specifically known as a requirement at the outset.

To consider what will be required for first fix electrics and plumbing, have a read of our handy checklist here too.


In the case of a new build and as referenced above there will always be associated costs for connecting you up with utility suppliers such as cable/broadband, gas, electricity, water and drainage.  These costs vary by location and ease of connection.  Where traffic management is required to support these connections and installations, these also come with an associated cost often well in to the £thousands.

Traffic Management installing water mains

Ordinarily these costs won’t be included in the builder’s quote as they are such a variable unknown, though they should be flagged as a cost you will need to carry.

I’d also add here not to underestimate the time that utility connections can take.  This should be a priority to organise with your build project as there is usually a lead-in time of several weeks, sometimes months.

Whilst your builder can point you in the right direction here and let you know what needs to be done, this is also invariably something that the customer has to arrange themselves.  This is because utilities are the responsibility of the property owner who is contractually and financially liable for the set up and future costs.

Change from Plans

Loft conversion

Until ceilings are pulled down and floors ripped up, where you then have full visibility of what is underneath/above, an architect generally has to guestimate the base from which they are working.  Therefore, both their planning and building reg. drawings involve a certain amount of guestimation.  Therefore, it is quite common that changes need to be made, or additional measures taken, to build what is intended. For example, until we can see what the sub-floor consists of, we can’t accurately advise what ground and floor preparation works will be necessary.  Until we start taking your roof off, we can’t see if your existing house roof is eroded and will need fixing, before we extend on to it.  Until we take your ceilings down or floors up, we can’t see which way your joists are going.  If we’re decorating a room and identify that the render is blown, this may need stripping and redoing.  Some of these things just can’t be known until we’ve ripped out what is already there for full visibility on our starting point.

With full visibility we can then identify any further unexpected costs when building a house.  Again, these should be clearly communicated by the builder to manage your expectations going forward.

Frequently, with full visibility, Mac has then overseen changes to steel requirements to effectively support the home.  Where a structural engineer is involved for calculating steel requirements this also has a corresponding cost both for his involvement and the outcome of his instructions in terms of steels required.

Size Matters

Believe us, in building matters, size does matter!  Builders will ordinarily quote based on size.  Mac will quote for a build project based on square metre size, so every sqm = £x cost.  In turn, many of our suppliers also quote based on sqm.  Therefore, if an extension or build is larger in sqm than the size allowed in the quote, this has a knock-on effect on cost that will be passed on. Insulation, underfloor heating, plaster-boarding….all these materials and many more have a cost per sqm too.  A surprising one that can sometimes cause unexpected costs when building a house is where sizes given to the builder have been underestimated.  We have seen this particularly where loft conversions are involved.   If the builder identifies that actual size of area is different to that communicated at the outset on which he’s based his quote, renovating/building the additional square metres will need to be added to costs.

Home Extension

Increased cost of Materials and Labour

Your builder’s quote will likely have a timeline or expiry date attached to it, or make it clear that the cost of certain materials may be subject to change.  If there is a long period between receiving your quote and starting the works, it is possible that material or labour costs may have gone up in this time.   Discuss this with your builder before works begin to understand if there are any changes expected to material and labour costs since your quote was received.

Furniture, Fixtures and Equipment (FF&E)

A builder’s quote won’t usually include your kitchen and it’s appliances, bathroom furniture and sanitary ware, radiators, staircase, internal doors, tiles, carpets and flooring.  ReVamp invariably provides customers approximate costings for these where requested as a ball park figure for your budgeting.

My advice here is always to allow good contingency.  As discussed above when talking about electrical and lighting finishes, these all make up the final aesthetic and interior finish of your home.  These are the items that characterise your home, personalise it, make it homely, and give it the wow factor if that’s what you’re after.  They also give your space function, making each area or room fit for purpose specifically for your family’s requirements.

Naturally, these costs come towards the middle or end of a build, often when budgets are more strained.  This can be challenging.  You’ve already invested so much up until this point, and the final finish is all in these details.  These finishes create the look and feel that makes a house a home.  More specifically, that makes it your home.

I therefore implore you to save a good budget for these final finishes to have the home you want and deserve relative to your overall investment.

Hallway entrance with karndean heritage collection flooring

Energy Efficiency Cost Considerations

When kitting out your home, be it with appliances, windows or insulation, there may be options that seem more affordable.   It’s important for long term cost implications (and the planet!) to consider the energy efficiency as well when making your product decisions.  For example, windows with a lower U value may be slightly pricier but they will be much more energy efficient so save you more money in the long term.  Likewise, a washing machine or fridge may be more expensive, but if it comes with a better energy efficient rating this could be extra money well spent.  I therefore advocate a longer-term view on products relative to their cost and efficiency.   Spending a bit more on energy efficient products at the outset could reap the rewards long-term in your cost of living and carbon footprint.  As the saying goes, you get what you pay for.


Garden Landscaping when building a house

Landscaping is not usually included in your builder’s costs, but it will be a necessity to finish off the external appearance of your driveway and gardens.    Of course, some of this could follow at a later date when you’ve had time to save up again, but there will likely be areas post-build that need to be ‘made good’ again.

You may also need to replace fencing or pathways shared with neighbours, restore public verges/paths that have been mucked up by all the trade vehicles, or make areas safe again.

A good builder should do their best to protect grounds such as driveway, laying protective surfaces over these if there are vehicles and skips coming and going on top.  However, you need to anticipate and allow for some landscaping repair work post-build, along with factoring any long-term landscaping and driveway plans you may have.

Imagine spending thousands on creating a beautiful home, then leaving the approach / entrance unfinished?  You know you’ll want to finish it all off, driveway, gardens and all to complete the picture. 

Although ReVamp don’t undertake landscaping ourselves, we work closely with some ground works, landscaping, driveway, tree surgeon and planting companies.   We can therefore invariably organise any landscaping jobs that need to be co-ordinated around your build project, or point you in the right direction to get quotes.   Your builder most likely has similar connections too.

A final point to also bear in mind with landscaping – it is good to discuss this with your builder in connection to your building works to consider what needs to be done in what order and how best to co-ordinate these around one another. 

For example, if we’re awaiting planning permission on a development, we’ll often crack on with landscaping the back garden in the meantime.  Infact, doing any back garden clearance and ground prep works ahead of the build can be very beneficial, as doing so during or after the build can create a lot of mess around the home or building site.   Particularly where side access to the rear is limited, getting any back garden landscaping done before or early in your build project would be recommended.   It could otherwise add unnecessary additional costs to do this further down the line if areas need protecting or access is challenging.


Yes, I know, it’s very British to talk about the weather.  However, being in Blighty, it is relevant.  Bad weather can slow down a build and bring additional unexpected costs when building a house.   Once your timeline is impacted by bad weather this can then have a knock-on effect on labour costs.   Also, if you’ve moved out and are renting a home whilst your building works are taking place, any delay is a direct additional cost.

If there are significant works to do that require good weather, such as roof work, then we’d highly recommend you put a tin roof over your house whilst works are ongoing.  This enables the trades to work on your build project regardless of weather.  If you factor in the scaffolding/tin roof at the outset, then it doesn’t need to be added to the list of unexpected costs when building a house!  On the other hand, should you choose not to put a tin roof over your home, leaving it exposed to the elements, you’re more likely to have unexpected additional costs if the heavens open and flood your home with rain water.

Scaffolding when building a house


Well, I hope I’ve not scared you too much with all the potential additional unexpected costs when building a house!  Although it can be a minefield and stressful, it should also be an exciting time for you.  Just go in to your project aware that additional costs will be inevitable.  We want everyone’s building journey to be a positive one and hope that the information provided here will help manage your expectations for a smoother journey and less surprises.

With a good, experienced and reputable builder on board, a reasonable contingency allowance and an understanding that surprises will come up along the way you should be well placed to handle the unexpected within your build project.   Good luck!

You may also want to read our blog here on checking out your builder so you have the upmost confidence in them!  If you’ve found this blog useful be sure to sign up with us so you don’t miss other posts, or follow us on Facebook and Instagram.

2 thoughts on “Unexpected Build Costs”

  1. Unexpected costs in home construction can be quite a surprise. From unforeseen structural issues to design changes, these expenses can add up quickly. It’s a reminder of the importance of budgeting for contingencies and working closely with professionals to minimize surprises during the building process.

Leave a Reply

Your email address will not be published. Required fields are marked *