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top 10 mistakes when building a home extension marlow bucksThe 'top 10 mistakes' some home owners can make during the build process of your home extension on site with your builder - A clients guide to building a home extension.


This guide assumes that you have utilised my services for all the design stages and the advice is based upon using my documents supplied.

This guide assumes that CHP will not be engaged for the ‘Contract Administration’.

MISTAKE NO. 1 - NOT OBTAINING A SIGNED TENDER RETURN FORM FROM THE BUILDER. This document is within the rear of the specification manual. Even if the builder submits his own separate written tender letter, you still need him to complete this tender return form as it confirms the details as to what he has priced for and included and that he has read and understands the tender documents provided. A separate tender letter from the builder is unlikely to state this.

MISTAKE NO. 2 - PAYING YOUR BUILDER CASH AND UPFRONT OF ANY WORKS PROVIDED. This will expose you to additional risk and conflict. Do not be tempted. Be very wary of any builder that suggests this method - even at a discount.

MISTAKE NO. 3 - MAKING AD-HOC CHANGES TO THE AGREED WORKS DURING THE BUILD - Try to avoid this if you can. Sometimes it is unavoidable especially if a homeowner has misunderstand the plans or specifications. If there are any changes, identify them early before being installed as this will mitigate any extra costs.

MISTAKE NO. 4 - FAILURE TO RECORD AND AGREE ANY EXTRA WORKS OR OMISSIONS AS WORK PROCEEDS - Tallying it all up at the end from memory will result in financial shocks and an uncontrolled budget with the usual builder fall outs when things are not agreed. Keep an ongoing written record of all changes and agree costs / savings on an item by item basis.

MISTAKE NO. 5 - NOT HAVING AN ADEQUATE CONTINGENCY SUM - Altering any existing property will reveal areas of unidentified work - that is the nature of the beast. Having a 10% contingency sum would be reasonable and useful.

MISTAKE NO. 6 - PLACING TOO MUCH RELIANCE ON THE PURSUIT OF COMPLETION DATES - Most bespoke building projects are wide ranging and complex with access to many trades at various times throughout the job. This can be hard to integrate within a smooth ‘no gap’ programme due to the nature of sub-contracted labour. It is better for the job to over-run a month and to get it right than to rush and cut corners simply to move in on a fixed date.

MISTAKE NO. 7 - NOT USING MY COMPLETE PACKAGE AS THE BASIS OF THE CONTRACT DOCUMENTS - Yes the specification manual(s) is a wordy document and yes builders love pictures but hate to read the detail but it is vital that the plans and specifications form the complete basis of engaging your builder. Simply think of the specification manual as a ‘shopping list’ of requirements and what to do if things go wrong. Not everything can be expressed within a drawing.

MISTAKE NO. 8 - NOT BEING 100% INVOLVED WITH THE PROJECT ON A DAY TO DAY BASIS - If you go away during the build or only attend the site randomly the builder and his trades will be making important decisions for you some of which will not be to your liking and by then its too late to put right or expensive to alter. You are normally called on daily to make decisions and choices especially during the first fix of the services. You will even spot things that you will want to query for peace of mind. Become involved on a daily basis but do not micro-manage.

MISTAKE NO. 9 - BEING SUSCEPTIBLE TO ‘BUILDERS BLUFF’ FOR EXPLOITATION OF UNFAIR EXTRAS - Many builders are ‘past masters’ at finding extras on a job and seeking additional monies for so called ‘unknown work’ or for having to ‘adapt standard installation procedures’ above what they would consider normal. They are also very good at identifying extras, but often forget to inform you of any savings of labour or materials that they may have also found. If you are in any doubt consult with the Specifications or contact CHP first.

MISTAKE NO. 10 - NOT OBTAINING THE COMPLETION CERTIFICATE FROM THE COUNCIL - Your builder is responsible for this. Do not provide final payment until this document this has been successfully obtained.

I hope this short resume of the building period is of use to you and has answered your questions that you may have had. It may not be fully conclusive in content and is merely a 'memory jogger' of some items. If it does not answer all of your questions, please do telephone my office and I will try to assist further.


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Architectural and Building Design Services - for planning permission and building regulations within Marlow, High Wycombe, Maidenhead, Bucks, Berks and Oxon areas.

Residential development for household extensions, conversions and new dwellings.

Measured surveys and roof inspection reports. Drawing plans on cad and specifications for planning permission, planning consent, planning approval, local authority approvals, building surveys and party wall act.

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