If the conveyance is done there may be very little for the solicitor to do.
I sense you have other things in mind, but in terms of the building work it depends on what you're doing.
There are several "flavours" of professional involvement and each will carry its own costs implication regarding the involvement of the solicitor. I realise this may not answer your question with a solicitors fee figure, but it really depends on what you're asking them to do. If you decide to go down the classical "Employer-Main Contractor with the contract administered by the Architect" route having already conveyed the land there many not be much for them to do.
If you're doing a self build but engaging an architect, then the architect will issue the tenders, advise on them, advise on singing the contract and the terms, administer the contract and issue certificates for initial and interim payments. You may not need a solicitor at all as the architect will advise you on nominated sub-contracts as well.
If you're using direct labour instead of a main cotractor, the position changes. Its possible several short forms of contract could be used but I would resist being asked to become involved in something like that - too messy in terms of adminstration overheads and potentially huge risk on safety grounds. In that case I'd be tempted to issue a set of working drawings and leave you to it.
In that case I think it would be the self-builder who engages the direct labour operatives using contracts forwarded by his solicitor, and making sure each operative undertakes to look after his own health and safety issues. The solicitor may have a role in the signing of such contracts.
If a project manager is involved to manage the direct labour it is possible he may issue the contracts on your behalf, but again, the solicitor may be invollved in drafting the contracts and executing them.
If you intend to avoid architect, project manager and solicitor, you are - quite literally - on you own. However any costs you may save could be more than offset at the enventualy sale of the property when the purchaser realises you cannot offer certification by a professional involved in the build.
The only other issue that springs to mind is the payment of contibutions to the local authority and again, these can be organised through your building professionals or by yourself or your solicitor.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.