I love to enable my clients and so I offer an accessible, as-needs service.
Often the amount of design work offered by a builder is very little. Builders by definition go straight to approval to build and only require drawings for that purpose. Design is a bit like art or music – regarded as discretionary, expected for free, a bit of pot luck, with rather intangible benefits, and perhaps something many people wish to undertake themselves.
I urge people wishing to build without a design phase to at least consider site analysis, concept stage design and some design development, and to use an architect before a draftsperson or builder is engaged to deliver your building.
I have found that many people wish to get an early idea of cost before they pay for drawings. Some design time is required for this, and it must come first.
A precise cost to build requires a lot of design and documentation. A broad opinion of cost can sometimes be offered early. If you don't feel willing to spend a lot of money on a thorough set of drawings until you know whether you can afford your building, then hourly rates or a smaller parcel of design work, by an architect, is a very good idea.
It is difficult to expect reliability and accuracy from any quote given by a builder with concept design drawings only as a basis; this is simply because there would be many items and details not captured in these drawings. That detail would be resolved with further drawing and documentation and would have a bearing on cost. In principle, my advice would be to not use sketch design drawings as a basis for obtaining costs from builders.
However, the one instance where it could be appropriate and reliable to use sketch design drawings is when the builder is involved from an early stage, has been able to help develop the concepts presented, and makes clear to you the assumptions made regarding quality of craftsmanship. In some instances this is my strategy for those clients wishing to gain early opinions of cost. Establish an early a relationship with a builder, but request that builder to participate in some collaborative design and budgeting - this could be via a design meeting.
Using just concept drawings to estimate price without awareness of all aspects of your brief will result in an inaccurate estimation of price, so be sure to explain in detail what you want to see in your build to any person interested in quoting or giving opinion.
The traditional method is to capture in more thorough documents a clear and fuller description of the scope of works and to tender this to multiple builders, by using many more annotations, images, specification for benchmarks for quality and materials sourcing.
A last alternative would be to engage a Quantity Surveyor, but, in my opinion, this is less preferable than a price offered by a party who will ultimately execute the work. A QS will use a safe method and apply a contingency appropriate to the level of articulation and resolution of the drawings provided.
A tender process ought to consider not just price, but also personal character including communication skills and capacity to deliver to meet the brief.
I understand many clients of mine are keenly interested in using budget to define and determine their next steps, so I encourage people to use the method which makes the most sense.
I can execute an A4 document in editable text format to give a layman’s description of some benchmark for quality and likely materials, to accompany a set of photographs of your existing house and a cover letter requesting an opinion of cost for your use with the concept drawings. This will help bring a level playing field to such an early stage tender/estimate, as will seeking more than three builders give a rounded overview.