Try to minimise them
Think about access. That lovely garden lot down a narrow, windy lane with overhanging trees, might be fine for you in your car, but lots of deliveries are cheapest on large lorries, generally the bigger the lorry the cheaper the delivery cost, but if he can only get 200m away from your site (or more) you'll be hiring a telehandler and driver to get the goods on site ... there goes any money you thought you'd saved!
Remember if you're arranging finance today, you've likely to still be buying materials in perhaps 18 months's time. Building materials tend to go up twice a year, in spring and again in autumn, so try to agree fixed prices on as much as you can. Due to various reasons, some timber and insulation products have risen by more than 20% over the last two and a half years, A few years ago there was a shortage of bricks, suddenly, they were like gold dust ... so agree a price, and reserve however many you need and if necessary pay a holding deposit.
Once you have planning permission, it's a good idea to take your plans to your builder's merchant and get quotes for high value items such as concrete, bricks, timber frames and roofing.
Costs may change as the design process continues and site conditions become a factor, for example your foundations may increase significantly if an engineered solution is required by Building control or your Warranty Provider.
Costs will be affected by the method of construction you select and the type of contract for the build as well as your own specification.
You should contact your preferred utility providers for estimated figures as costs for service connections are site specific and can vary significantly between providers.
Be aware that if Detailed Planning Permission has not yet been granted for your project, conditions imposed by the planning authorities at a later date can affect your costs. Contractors are unlikely to provide a fixed price quotation until after Building Regulation aproval has been gained.
Try to minimise them
Create a 'must have' list and a 'wish' list
A £15 door opens as well as a £250 door