As a very rough rule of thumb, in average parts of the UK the cost of house building (excluding the land and excluding design fees) will vary between about £800/m² for a very simple design with no frills up to about £1,800 for a house with quite a high design standard and a high specification of materials and finishes. £1100/m² might sit in the middle. Of course it can go way higher than all this if you get carried away with your imagination!
As another rough rule of thumb, half of this cost will be accounted for by materials and half by labour. It is strange how these two tend to be roughly equal.
All these costs will increase by maybe up to 25% in areas such as central London and will decrease somewhat in cheaper parts of the country. The cost per square metre is based on the total floor area inside of the external walls and includes the internal walls area.
This is the sort of quote builders might come up with if you asked them to do all the work. Depending on how much of the organising and manual work you do yourself, the cost will reduce proportionally.
Architects fees will be around 7% for new build and 10.5% for existing buildings and there will be other fees. (that is as a percentage of the total building cost including labour and materials. The cost of land is of course an entirely separate matter and nothing to do with building costs or professional fees.)
There will be extra costs for any ‘features’, green or otherwise. These can vary tremendously. The figures from the Passivhaus institute indicate a very small increase for PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. construction of between 2% and 4%, partly because a Passivhaus is so energy efficient that it does not need a central heating system and therefore makes a saving which is offset against the higher amount for insulation (however Passivhaus construction is not yet established in the UK so the costs of an innovative standard could initially be considerably higher). Extra insulation is almost always a low expenditure item. At the other end of the spectrum[for the purposes of part C of the Approved Documents] - The composition of a particular sound in terms of separate frequency bands. is photo voltaic solar collection which might easily be £15,000 to £20,000 for a 2kW installation, in other words more than a 10% increase on costs.
This is a useful way of getting an initial idea of the cost of the project but unless money is no object it makes a lot of sense to do detailed calculations on the whole job before starting. However, you (or your architect) can employ a quantity surveyor to do more accurate costings once design work is in progress.
The Homebuilding and Renovating web site forum gives an indication of the wide range of costings that people come up with. There is also the what Pric£? site where there are quite a lot of prices people have actually been quoted on different types of work
When it comes to pricing fairly standard items, such as brickwork, trenches, pipework, etc. it is possible to use pricing books such as Spons but for anything slightly unusual you need to get quotes from subcontractors.
There are various on-line companies which provide an estimating service such as estimators-online
With most types of domestic self build you can claim back the VAT on building materials at the end of the job and this can affect how you do your budgeting. However there are several exceptions (such as a granny flat) and it pays to read the HM Revenue and Customs guidance notes on this.