Building your dream home extension

Building your home extensionbuilding the dream

With the formalities such as town planning, building regulation approval etc. dealt with it  may be time to consider actually having it built.

There are numerous approaches to building a home extension and we have outlined some of the more common ones although numerous other variations are possible. The cost of home extensions may vary to some extent with the various methods.


It is quite rare for anyone to do a full DIY extension but doing aspects of it are more common perhaps where one is skilled in one of the trades. Some people have the main structural shell built by a builder but then finish the internal works such as insulation, plasterboarding, plumbing etc. themselves. This can often save considerable amounts but bear in mind that finishing touches are often the ones that other people notice most if not done well. A further possible difficulty with  this approach is that the final ‘signing  off’ by the  building inspector may take place long after the builder has left the site and been paid so could cause difficulties if problems come to light at this stage. In these circumstances it would be advisable to ask the building inspector to look at the works at the stage when the builder has largely completed his part to see that there is no significant rectification required.


Most builders will subcontract at least part of the work, often the more specialist trades such as electrician or plumber  will be dealt with this way but sometimes the major trades including bricklayer and carpenters depending upon their in house expertise and what other jobs they be dealing with at that time. Effectively the main contractor is acting as what has become a popular title the ‘project manager’ (although there are individuals and organisations that specifically offer this service). One is paying for this expertise plus their profit and overheads in running the job. Some people choose to bypass this by directly employing contractors and buying the materials themselves (other than some trades such as electrician who on this type of work would generally supply the materials themselves). This has the potential to save money but it is not without its problems particularly for those unfamiliar with the building industry and its procedures. Some points to consider!

  • The overall co-ordination of the work and ensuring the right materials are on site at the right time will be down to you!
  • It can be difficult to locate good subcontractors who deal with this type of work, although once you have they are often a good source of recommendation for other trades
  • It is sometimes difficult to know whether you are being quoted a reasonable price unless you can obtain sufficient alternatives for comparison.
  • With materials the internet can be useful check on prices even if you end up using a local supplier.

General contractor

Most domestic extensions are probably carried out by a builder who will do part and sub contract the rest of the work and effectively project manage the contract. This will normally be for a fixed sum (subject to changes etc. that may take place) so makes it easier to budget for.

General contractor with contract administration

This is a variation of the above where instead of the householder dealing directly with the builder a professional (usually the architect or designer but sometimes a separate organisation such as a surveying practice) will deal with aspects such as preparing the tender documents, obtaining quotations and then inspecting the works and agreeing interim payments. There will obviously be additional fees for this (and sometimes the builders quotes may allow for a higher degree of supervision) but can be useful for clients not happy in dealing directly with builders or for complicated or large schemes.

Design and build

This is where both the design and construction are effectively dealt with under the same contract, it may sometimes use in house design expertise, other times the building contractor may use an external design consultant but commission them directly. It may be more difficult to compare schemes from competing firms as the detailed design will probably not have been produced at that stage unlike when builders quote against detailed plans. It tends to be used in the more specialist areas of home extensions – loft conversions, basements, prefabricated home extensions, conservatories etc. where their particular expertise can be incorporated at an early stage.

Choosing a builder

In reality most builders are neither rogues or saints so whilst personal recommendation is the best approach it needs to be in relation to a similar type of job, a builder who largely deals with a modest suburban extensions may be out of their depth with a multimillion pound  extension and refurbishment to a listed building. Try to see examples of their work and ask previous clients what their experience of them was. However, even with similar types of work experience can vary. To some extent it depends upon the relationship between the builder and the client so it needs to be someone you are comfortable dealing with, it does not need to be on a ‘chummy’ basis – in fact it is sometimes better if it is not, but should be open and courteous.  Many builders dealing with home extensions are often small companies with just a few employees (although some are literally a ‘one man band’) however some medium sized organisations may have small works departments to deal with this type of contract.

If you are not able to obtain personal recommendations then it may be worth mentioning possible candidates to building inspectors, designers etc. who may have an opinion if they have dealt with them. Often building inspectors and other officials are reluctant to give actual recommendations but none the less their reaction when some names are mentioned speak volumes!

There are directories such as Yellow Pages, Thomson which lists numerous builders although this can be like looking for a needle in a haystack. There are organisations that some builders are members of such as a Federation of Master Builders ( and NHBC ( but some of these may be more for the benefit of members or not strictly applicable to extensions (the NHBC is primarily concerned with new housing). None the less there are some schemes that may be worth noting such as Trustmark ( ) which is government supported and has some degree of monitoring members. Some of the commercial websites such as Checkatrade ( give some feedback from previous users.

Avoid any builders who will only provide a mobile phone number and are elusive about their address.

Obtain quotations from around three builders, bear in mind there can be a lot of work involved in preparing a quote so do not get too carried away with obtaining them. They should be termed quotations, estimates are really only applicable if the nature of the works are uncertain. Quotations should clarify whether VAT is applicable, which for most works it will be but see Building Costs and VAT. The more information that can be provided in terms of plans, specifications etc. the more accurate a builder can work out a price and the less chance there will be of disputes at a later stage. Although the temptation is to take the lowest quote, unless the difference is large it may be better to go with the one who you feel happiest with in terms of dealing with or their standard of work. If there are significant differences in prices it may be due to deliberate under-pricing in the hope of making it up on ‘extras’ or overpricing can be due to builders not really wanting the job but feeling obliged to quote. Neither is an ideal basis for a building contract.

Building contracts

There is strictly speaking no need to have a formal building contract as it can be legally binding even if only verbally agreed (although rather harder to prove). Generally contracts made in a customer’s home will be subject to a seven day cancellation period. None the less if something goes wrong it is generally better to have it clearly laid out what was agreed and what steps to take where there is a dispute. Several types of building contracts are by JCT (, where there are professional supervision contracts such as JCT minor works or HO/C might be used or HO/B without it. Other contracts are produced by organisations such as the Federation of Master Builders or consultants may have their own although ensure they are fair to all sides.

Payments to builders

Most home extension projects go on long enough for it to be usual to make interim payments, maybe weekly or monthly or when certain stages have been reached, depending on the project. Avoid paying in advance for anything other than a nominal deposit, bear in mind established builders will buy most of their materials on account which are paid in arrears so look sceptically at requests ‘for money to buy materials, on the other hand do not unreasonably delay payments, often the amounts invoiced are quite large for what may be a one man business, builders have their share of financial difficulties. Where it is sometimes unavoidable to pay for something in advance such as for specially made items, look into the possibility of paying by credit card or through a deposit guarantee scheme.


We have become accustomed for large items such as cars, new houses etc. to come with a formal guarantee. In relation to home extensions this is rather rare although work will to some extent be covered by normal consumer protection legislation. If problems occur formal guarantees are only of use if the company is still likely to be trading. If you need to claim so other than with large and stable organisations an insurance backed guarantee is more desirable. Some builders operating through schemes such as Trustmark or the Federation of Master Builders may be able to offer these.


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