Due diligence on a development site is not the same exercise as due diligence on a house. You are not asking whether the building is sound. You are asking whether the thing you intend to build can be built, at the cost you have allowed, in the time you have assumed. That is a different list of questions, and it belongs in site sourcing and appraisal rather than in a valuation.
The seven items below are the ones that most often turn a workable appraisal into an unworkable one. None of them is exotic. All of them are survivable if you know about them before you commit, because then they are either priced, negotiated or avoided.
1. What is under the ground
Ground conditions decide the foundation solution, and the foundation solution is one of the few parts of a scheme where the cost range is very wide. Made ground, a high water table, soft alluvial deposits, old foundations, tanks, obstructions or contamination each push the design in a different direction, and the difference between a simple strip footing and a piled solution with a capping beam is a material number on any scheme.
A desk study costs very little and tells you what the historical maps, geology and previous uses suggest. An intrusive investigation costs more and tells you what is actually there. On anything other than a straightforward greenfield plot, the second one is worth buying before exchange, and where the vendor will not allow access, that itself is information.
Contamination carries a second cost beyond the remediation: time. A remediation strategy may need agreement with the local authority, and the programme allowance for that varies by council.
2. How you get in and out
Access governs the method, and the method governs the price. A site a lorry can enter and turn in will be built differently from one served by a single narrow entrance off a busy road. Crane position, material storage, welfare accommodation, delivery hours and whether the pavement or carriageway has to be occupied all follow from it.
Where access is tight, the contractor prices logistics rather than just construction: smaller deliveries, more labour, hoists instead of cranes, out-of-hours working. Those allowances sit in the preliminaries and they do not reduce when the drawings improve.
Rights of access over land you do not own need to be checked by the solicitor, not assumed from the site visit. A track that has always been used is not the same thing as a right to use it.
3. The boundaries and the neighbours
Work near or on a shared boundary brings the party wall process into the programme. Excavation close to a neighbouring foundation, work to a party structure, or underpinning all require notices to be served, and the neighbour has a right to appoint a surveyor. The process is well established, but it takes time and it can change the method.
A schedule of condition on the adjoining properties before work starts is worth having whatever the legal position, because the alternative is arguing about which cracks were there first. We set out the practical side in party walls and neighbours.
Where the scheme is tight against a boundary, check whether the construction method needs access to the neighbour’s land at all. If it does, that is a negotiation with someone who has no obligation to say yes.
4. Services, capacity and diversions
Two separate questions sit here. Is there anything crossing the site that has to be moved, and is there enough capacity for what you intend to build. The first is a cost and a programme risk. The second can stop a scheme entirely.
Diversions of water, gas, electricity or telecoms are carried out by the relevant undertaker on their own timescale, and the quotation process is slow enough that it needs starting early. Sewers crossing the site may require agreement and may dictate where the building can sit at all.
New connections and upgraded capacity, particularly electrical supply, have lead times that vary by provider and region. On schemes with significant electrical demand this is worth a direct enquiry rather than an allowance, because the answer sometimes changes the design.
5. What the planning history tells you
The planning record for a site is public and it is one of the cheapest pieces of due diligence available. Refused applications tell you what the authority objects to. Approved applications tell you what it will accept, and the conditions attached tell you what it will insist on.
Where consent already exists, read the conditions and the obligations rather than the decision notice alone. Pre-commencement conditions have to be discharged before work starts and each discharge takes time. Obligations may carry payments or works that belong in the appraisal.
Whether a particular route is available for your scheme depends on the authority and on the site, so the answer has to come from the council in question. For conversions specifically, the choice of route is worth deciding early, and we cover it in office to residential and prior approval.
6. Title, covenants and what you are actually buying
The title can restrict what you build even where planning permission allows it. Restrictive covenants limiting use, height or the number of dwellings, rights of way across the site, easements for services, ransom strips at the access point and unclear ownership of boundary features are all common and all capable of changing the scheme.
This is the solicitor’s territory, but the commercial consequence is yours. A covenant that caps the site at one dwelling is not a legal footnote when the appraisal assumes four. Ask for the title early and read the plan against your proposed layout.
7. Rights of light and overlooking
A neighbouring owner may have an established right to light through a particular window, separate from anything the planning system considers. Where a scheme reduces that light below the protected level, the consequence can be a payment, a design change or, at the extreme, an injunction.
A rights of light assessment on a constrained urban site is a specialist report and it is worth commissioning before the massing is fixed, not after the planning application has been submitted. Planning consent does not override a private right.
Overlooking and daylight inside your own scheme are planning matters and they belong in the design brief, which is a separate exercise set out in planning and design coordination.
What to do with the answers
Each of these seven either prices, changes the design, changes the programme or changes the price you offer for the land. That is the point of doing them before exchange: while all four of those responses are still available to you.
Where an item cannot be resolved in time, the structure of the deal can carry it. A conditional contract, an option, a longer completion or a price adjustment mechanism are all ways of buying time to find out, and all of them are easier to agree before you have committed than after.
The cost consequences then need measuring rather than estimating. An allowance for abnormals that is a percentage of the build cost is a placeholder, not a budget, and turning it into a real figure is cost planning and estimating work. The wider programme of checks sits inside development management.
About to exchange on a site?
We review what you are buying against what you intend to build, and price the items that will not go away.