Written by Emily Scoones on 14/05/2020

The Importance of Cost Data in Site Appraisal

It is hugely important to have the full picture when understanding the potential of a site. This is exacerbated on marginally viable land around transport hubs where a site’s viability can be predicated on a range of site-specific complex factors. Costing these abnormalities is a crucial part of understanding the potential risks and opportunities of building out those sites. At an early-stage, simply basing the number of units achievable on policy-based density matrices, in isolation from this information, can lead to the wrong strategic decisions being made wasting time and money.

In our recent Mass Land Viability study for Transport for London (TfL) to appraise a large volume of potential development sites in London, cost played a vital role in ensuring better outcomes. Here, the SiteSolve team collaborated with Turner & Townsend (T&T), who provided detailed cost data through their new Parametric Cost Modelling API.

We caught up with T&T’s Cindy Curia to ask her some questions on their new system.

How does your system in this process compare to the level of detail you would usually supply at this stage of design?

CC - "Turner & Townsend’s Parametric Cost Modelling (PCM) solution is aligned with RIBA Stage 0-1 cost advice. Once adapted to our clients bespoke dataset, the PCM solution provides detailed, relevant cost advice within a matter of milliseconds."

What is your goal of having this API and hub of data?

CC - "The PCM has been developed in response to our clients greatest need at the early stages of design - providing quick, relevant cost advice for multiple design options. Working with Ramboll on the TfL exercise, Turner & Townsend’s PCM API (hosted on our proprietary cloud platform) priced millions of building options generated by Ramboll’s SiteSolve system, producing a RIBA 1 Cost Plan every 50 milliseconds. This has huge implications on how exercises are traditionally carried out and offers our clients accurate cost advice with the click of a button."

What is the biggest challenge/risk and opportunity of using cost data in this way?

CC -  “Encouraging early adopters; the PCM is a powerful tool that provides the same level of detail as a traditional RIBA Stage 0-1 cost plan with speed. Challenging the project team to take advantage of this tool early on in the design life, will support the team finding the most valuable and efficient solution for each land parcel. Having this knowledge early on will avoid costly changes and time lost finding efficient layouts later on in the project life.“

For each massing SiteSolve created, the tool sent a detailed breakdown of the key area data and site specific information, such as the area of a the site in a flood zone or with steep topology and the area of façade that is next to a railway, directly to T&T’s API. Within milliseconds, the site was costed to a greater level of detail than blanket cost per square metre which is crucial for sites with abnormalities.

Curia noted “Abnormal costs are prevalent in the early stages on design where site investigations have not yet been completed.  These could include ground risk, existing utility capabilities and location factors (noise and vibration). Harvesting our data lake of project knowledge, Turner & Townsend are able to build algorithms to calculate site abnormals specific to the site location and existing use.”

This data, in combination with apartment values, enabled the SiteSolve optimisation algorithm to try to maximise return instead of the number of units on a site. The impact of including cost and value data could instantly be seen on some sites when comparing the output to the initial optimisation runs for maximising the number of apartments that could fit on the site.

The site pictured below shows an example of this impact. The adjacency to a railway, steep topology and inefficient single-sided building layouts meant maxing out the lettable area was not cost viable, however, optimising for return yielded a positive solution. More than that the site may have been considered as having high potential value based purely on a policy-base density model which identified significantly higher number of apartments. This may have resulted in further work on a site which, when explored further, would have proven to be unviable and not produce the number of homes anticipated.


The early use of more detailed cost data and technology can more accurately identify the sites which have the most potential to create valuable housing developments. It can de-risk sites that may initially look promising but also create opportunity by identifying sites which may have been deemed unviable. It can be used to find those tipping points where exceeding a certain height may be detrimental due to knock-on effects to the net-to-gross because the buildings need three lifts not two. And with that, we have the ability to prioritise and unlock more land in cities more efficiently and with greater certainty.