Top ten hidden costs of renovating and extending

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Read on, be prepared

While you may likely have developed a build budget for your refurbishment or extension there are some commonly overlooked costs that can significantly contribute to the cost of a project and that you may not have factored in yet. Not all will apply to your build, and your reputable architect or designer will be able to advise you in advance, but here are the top ten hidden costs of renovating and extending that you should be aware of.

1. Survey

In my quest to give owner-builders and renovators as much ownership of their project as possible, I always recommend owning your survey. This means that when your architect includes a survey as part of their costs, you specify that this survey should be independent of them. What this means if that should things go sour (which I very much hope they don’t), you can take this survey to a new architect, and it holds independent liability to the surveyor. If your architect ‘owns’ this survey, you have no rights to it (even though you have paid for it). Even if they are prepared for you to have a copy, you will have to pay for a new survey from your new architect, as they will not be prepared to accept liability’s in case there are errors in the survey which later impact the build. It may be that an independent survey costs you a little more (or it may not) but it is worth it for the value of the product.

There are also other common surveys which may add to your costs: foundation and structure examinations (if you are building on existing), drainage surveys, asbestos survey (which may be made a condition of planning permission approval, if asbestos is suspected), and arboricultural reports ( required if you have a tree preservation order (TPO) but which also may be made as a condition of approval, by the tree officer).

If you are extending downwards you may need an archeological report (likely if you live in an historic area) and ground condition assessment to establish how the new basement should be constructed.

Rather depressingly, the list goes on. It is always best to check with your local council or state of you are unsure about which surveys you will need for your plot/house.

2. Planning permission

If your build is outside of permitted development you will need to apply for planning permission. Alongside the professional fees to get you to this point, your local council will charge fees. Currently (as at January 2019) this stands at £206 for either full planning or for a lawful development certificate, for a residential permit*. If you are requesting permission to convert between uses, or change volume of occupancy, the fees will be higher.

3. Structural costs

Within your build fees, your tenders will include the cost for structural elements and works, however you will need to instruct and pay for structural engineers (independent of your architect or designer) to engineer the elements of your build which require support (knock throughs, basement construction). Fees vary depending on complexity. You will not only need these calculations for the builder, but you will also need them to apply to your council’s build control office for their approval.

4. Build control

Law requires you to employ a build control officer to oversee your builders’ works. You can either do this through the council, or privately. The costs are very similar, but the private officer, who must be accredited by the council, is likely to be more flexible about coming to site when required. Regardless of who you decide to go with, your council is the governing body.

Your build control officer is responsible for informing the council when you build starts, approving the standard of the ongoing build, and will provide you with your completion certificate at the end of your works. This certificate is also lodged with the council, and marks the standard of works done. This certificate is very important household document and should be retained as it will be needed should you rent or sell your property.

5. Party wall agreement

This is the crux of many a build in our close-living community. If your build or renovation links or impacts in any way with a joined house, or the boundary to a neighbour, you need to investigate having a party wall agreement. Each side can appoint a surveyor (at your cost) to clearly assess the condition of the existing structure, in case of future issues which may (or may not) be caused by your build works.
This could include, work to a “joining” chimney breast, roof extensions, side wall extension etc.

6. Listed Building Consent, Conservation Area Consent

If your property is listed or is in an area of outstanding national beauty or interest (if you are unsure your council will be able to inform) your planning permission will not go to the standard Planning Office at your local council. It will instead need to go to the appropriate Heritage Department. They will ensure that you are protecting the relevant historical parts of your property through documentation, identification and preservation. This application actually does not cost, however the architects fees to compile will be significantly more than a non-conservation application.
For reference, your property could be listed internally and or externally, and in a grading from 2, 2”star”/2* or 1, with 1 being the highest listing. The higher the listing the less charges you are permitted to make.

7. Often missed build costs

These are elements which are often included outside of tender in a “well, since we are doing all this, we may as well x,y,z…” sort of way. While they are easy to agree to, and they may seem small elements, they add up and should be considered as part of the initial tender:

  • Full replumbing – especially if your property has already been extended, differing sizes in pipes and differing materials can lead to a spaghetti junction affair where leaks are likely);
  • Full rewiring and/or renewing the fuse board – if your electrics won’t support additional ring mains, or your fuse box cannot accept more;
  • Levelling floor heights (if existing levels are uneven, but also if there are changes in overlays in a single storey);
  • Insulation – both sound and heating. Heating is more often thought of, but consider if you have two bedrooms adjoining, or a bed and a bath, do you want noises transmitting? Acoustic insulation can be easily added at build point, and should be considered as routine;
  • External coverage – do you need a “hat and coat” on the building while you do the works? This protects your project (and the builders!) against rain, snow, and moderate winds and can mean works happening over-winter which would otherwise be prevented by inclement weather;
  • Skip hire and permits – if your skip is parked on the road, you will need a license to do so: your project will be fined if you don’t have this in place;
  • Drainage – you may need to rest an existing manhole cover, re route external drains, or renew an existing system. If you have communal drains (common in terraced housing) which will be detailed on your deeds, you will need approval included in your planning permission from your local water provider ;
  • Guttering – if an extension interrupts existing downpipes you will need to consider re-routing this excess.
8. Landscaping

It almost goes without saying that not only will build works most likely disrupt your existing garden through access and storage, but if you are extending, your garden will need a touch of planning to fit the new build shape and use.

If you are having soakaways, consider where these will be best placed. If you need new light, power or water, plan early for this so that it can be integrated into your build works, and included as part of the costs.

Implementing the required protection for trees or their drainage area must be costed, if you have required an arboricultural report, as will temporary tree protection while you complete the build.

9. Fixtures and finishes

This is often the area which gets a stiff ignoring! You WILL require flooring, a wall finish, sockets and light switches, radiators, lights, skirting boards, doors… Not to mention a kitchen and bathroom with all that they entail. This is before we get to something to sit on, eat off, or open doors with…

Finishes cost between 20% and 40% of build costs.

10. Last but not least – contingency

An amount (ideally 20%) of your build costs should be allowed as a contingency. This is your in-house ‘insurance’ against unseen costs you choose, or the build revealing itself as less amenable than you had hoped. If this ends up as being surplus to requirements, great! You can have a big party! You do not have to disclose this amount to anyone, but you should have a figure set aside as a resource and to give peace of mind.

I hope this guide is helpful, not daunting. Knowledge is power. Now that you know the top ten hidden costs of renovating and extending you can plan for them. You have the power!

If you’re at the beginning of planning a project, you might also like to take a look at my planning timeline and extension timeline to give you an idea of the time scales involved.

* For current planning permissions fees in the UK, please visit the Fee Calculator at the Planning Portal here.


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