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  • Davide di Martino

How long before we can start building my extension?

After thinking for weeks or even months about finally starting your construction project, most people can't wait to see it taking shape. But along the way are many constraints, decisions, approvals and agreements that will take time and, unless properly programmed will ruin a lot of the pleasure and excitement a real project should entail.

Architects are taught to be very careful about projecting timeframes: I remember reading, among a list of questions that typically arise when discussing with an architect the fatal: “Will we be in by Christmas?”. The answer suggested was “Which Christmas?”. In other words, do not promise what you cannot guarantee. On the other hand, people will want to plan ahead, and part of the pleasure of a project is visualising the process (we definitely encourage all clients to do it!). So here is our advice on the typical timeframes for a medium-sized extension. It will not be a clear-cut answer, because at every step there is the risk of a mistake and a small setback. The good news is that for every risk there is a strategy, and we can do a lot to speed up the process, and most importantly, enjoy it.

The short answer: timeframes depend a lot on the type of project. For a complex extension with bespoke details, we usually factor in seven to eight months from appointment to starting works. More simple projects can take one or two months less than this.

Starting from a standard seven-month benchmark for a rear extension project, we can adjust the estimate based on a few more factors:

  • Type of project. The more complex and ambitious the project, the more time it will take. Ambitious projects tend to take more time to be designed, coordinated and granted planning permission.


Approval will typically take 10 weeks. Even though the maximum time allowed should be eight weeks, Councils often take one or two weeks to validate the applications. In the last year or so, Councils are requesting extensions of time more and more often. You should therefore factor in twelve weeks and be prepared to wait fourteen!

In order to reduce the impact of such a long gap between design and approval, and to reduce the risks of having to go back to the design board with a lot of work (and time) invested, we often resort to use the pre-application advice. That is, anytime our project is slightly different from the most typical examples, we approach planners to request written advice from the based on our preliminary design. The potential impact of this strategy can be both of saving time and money. You will save time, if we can get positive, or negative, feedback from the planners we given a steer on what will be successful when put to official planning and save time on the design side, it might add a month to the whole process but we believe this is easily saved elsewhere.

In the case of planners opposing the development at this stage (we would advise of this risk, of course) and a client may want to risk the official planning submission in any case. If a planning application is refused permission, you may need to invest ten more weeks in re-submitting and awaiting response on the new submission. Or, you may decide you want to appeal the decision, in which case you can expect to wait five to seven months for a decision. A lot depends on the degree of details and therefore certainty, you want to have before starting.

Finally there are special case which could add time to a development, projects involving basements and especially in certain boroughs (Camden, Kensington and Chelsea) or flood risk areas, conservation areas, or listed building, may need to meet more stringent requirements in terms of tree protection, dust control, safety of the excavations, drainage.

Design and detailing

Put simply, complex design require more time to elaborate and detail. They might require specialist subcontractors for a particular type of cladding or floor finish or structure. They will require larger, more sophisticated builders and more time to prepare and compare prices.

Degree of control:

  1. Tendering: A long tendering process, for example can save a lot of money, but also take quite some time. Most possible extras and contingencies will be explored so that the client has a clear idea of the likely final cost of construction. A long tendering process may also allow to save some time further down the line, if it provides time to clarify any construction uncertainties and order every long-lead item.

  2. Detail: pretty much connected with the above, more detailing takes more time but affords better results and more certainty about timeframes and costs during construction. A very detailed tender package will take about six weeks to prepare, compared to the two weeks required for a basic building regulations only set of plans.

Ways to save time if you are in a rush.

  1. Take risks in the planning stages. Some clients might be willing to take risks. In the planning stages, for example, we sometimes submit an initial design for pre-application advice which saves about six weeks on the standard programme. In case of a positive feedback, we then prepare the planning drawings and start the detailing at the same time. We don’t wait for the planning permission to engage in the detailed design because we assume the permission will be granted. There is a risk in this case that the permission is refused and we need to start detailing again, which would cost time and money.

  2. During tendering: Not tendering: some clients have a preferred builder and want to avoid tendering. There is a degree of risk here in potentially not getting the best price, but also that the builder is suddenly unavailable.

  3. Less detailing and management will take less time: the risk is transferred to the construction stage which can drag for longer and cost more.

What could go wrong?

  1. The tendering process is not successful: we do not find a builder we like, who is also available and within our budget. Or the builder we select drags the start of works or is suddenly unavailable and we need to start all over, wasting four to six weeks.

  2. The planners are extremely slow in responding to a request for pre-application advice or in judging a planning application.

  3. The neighbours strongly oppose the works and delay the preparation of the party wall agreement.

All in all, we suggest to begin with a seven to eight months' scenario, and navigate design and strategic options with our advice in mind.

Our final tip is to face the design stage as one of the best parts of the process: together with your architects and your family you will enjoy the virtual omnipotence of the designer, the pleasures of teamwork, the exhilarating experience of seeing the project take shape (perhaps a physical model!), while being knowing that things may need to change and flexibility is the greatest endowment of the contemporary designer.

If you would like to discuss a project with us do get in touch


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