Once you have finalized your plans and selected all your finishes (sanitary fittings, heating system, rooflights, doors, joinery finishes etc.), you can compile a definitive cost estimate.

Providing there are no major variations or changes made during the build, the figure you come up with in this estimate should not vary greatly from the final cost figure at the end of the project.

Converting an attic can prove an exceptionally good financial investment but the costs run to more than just the basic construction costs. Design fees, additional labour costs and wastage of materials are just some of the things that can trip you up during your project.

With Attic Conversions, the natural inclination of the discerning self-builder is to use materials that blend into the original. This means that the choice is often to use second hand or re-claimed materials, and these often carry a premium.

However, they also have a high wastage factor making it necessary to over order in the first place, as it can be very expensive or impossible to obtain extras in small quantities.

Labour costs may also be increased. For example, slates and tiles might well need re-drilling or shaping and this too slows down the work, therefore increasing the price.

The best way to control extras is to make certain that absolutely every detail is finalised and described accu¬rate¬ly within the plans and specification, and then to make absolutely no additions or alterations. This means choosing all of your materials, fixtures and finishes in advance of getting in quotes.

In reality, this is rarely practical and even if it were, it is unlikely that changes can be it is often impossible to turn down ideas that avoided altogether – will make for a better home – and so it is a good idea to allow for extras within your budget conting¬ency of 10%.

When you agree a contract with your builder, discuss the basis on which extras will be handled. When you ask a builder or subcontractor to do extra work, agree a price for labour and any materials there and then.

Whilst Builder’s Merchants normally include delivery within their price, any materials ordered direct from manufacturers or salvage yards will usually incur a delivery charge.

Make sure that when you get prices for your budget that the quote includes delivery. If your site is difficult to access, there may be additional delivery charges for staged deliveries on smaller vehicles.

Bulk materials such as bricks, blocks, pavers, multiple bags of cement and plaster etc. are usually delivered on wooden pallets.

Most builder’s merchants and direct suppliers charge extra for this and then say that this is refundable if the pallets are returned in good condition. The problems that then arise concern keeping the pallets in good condition – they make excellent firewood – and relying on the companies being available or close by to pick them up when required.

If there are delays on site and the hire period needs to be extended then there might well be a weekly penalty of around 10% of the total hire charges.

Perhaps the least obvious cost of all is that of your own time and expenses in visiting plots, the site, suppliers, architects and so on. The additional motoring costs of driving to and from the site every day can be considerable, as can the cost of the inevitable telephone calls, particularly on mobile phones, to builders and suppliers.

There is also a notional cost attached to your own time – which could perhaps otherwise be spent doing overtime, freelance work, or a second job. This is especially the case for DIYers putting in countless hours.

As soon as the project is under way, keeping tabs on the ongoing costs is crucial. Keep in touch with your tradesmen and remember, if you change your mind about things as you go along (which is quite normal) it will cost you more than the quote (again, normal and reasonable).

The trick is to be up front and ask what the extra costs are going to be. If you don’t, when the bill comes in and it’s 20% over the estimate, because of changes you asked for but didn’t get quotes on, you’ll be on the back foot.

Keep a spreadsheet on your computer. The moment you see that you are overspending you must cut back. In fact before you spend the money make sure that you do not go over the budgeting calculation. If necessary find another supplier.

You can soon forget who said what would cost how much. Keep good records, always getting your quotes in writing and check them off one by one to make sure you’re getting the right deals.