Change of use is the real trigger
A renovation retaining massing and use is generally consent-free in Germany and is subject essentially to the requirements applying to the elements being altered. As soon as the use changes, a different logic applies: the building is measured against the requirements of the new use, and not only where work is being done.
That is the key sentence of this article. A change of use triggers a reassessment of the entire building, including elements intended to remain untouched.
Typical cases with substantial step changes in requirements can be named. They almost always involve a change in the class of use.
| From | To | What newly applies |
|---|---|---|
| Office | Residential | sound insulation, daylight, second escape route, parking |
| Residential | Commercial or practice | accessibility, parking, in part special building requirements |
| Storage | Occupied space | room heights, daylight, ventilation, fire safety, escape routes |
| Loft | Living space | room height, structure, escape route, thermal performance |
| Agricultural | Residential | effectively the entire catalogue of requirements |
The cost of a change of use arises predominantly not from the conversion work itself but from these step changes in requirements. They concern fire safety, escape routes, sound insulation and accessibility at once.
The four risks governing dispersion
| Risk | What it triggers | When it resolves |
|---|---|---|
| Structure | strengthening of floors, beams or foundations: the largest single cost step in existing buildings | with the structural assessment including opening up of elements |
| Fire safety | escape routes, fire resistance periods, compartmentation, building class | only with the consent |
| Contamination | investigation, removal, disposal, plus protective measures during the works | only after the fabric is opened up |
| Building services | connected loads, service routes, ventilation, sanitary provision; relocating risers cuts into the structure | with the survey of the existing risers |
The special case of office to residential
Converting office buildings to housing is the most discussed case at present and the one where expectations are most often disappointed. The reason is that although both uses are occupied space, their requirement profiles diverge at decisive points.
Four points govern feasibility. If one of them fails, the conversion is not more expensive but simply not deliverable.
| Point | Where it fails |
|---|---|
| Building depth | living rooms need natural light; internal zones cannot be used as habitable area |
| Sound insulation | requirements sit well above office standard; retrofitted build-ups add weight and feed back into the structure |
| Sanitary risers | few central cores in an office against connections in every dwelling; new risers penetrate floors |
| Facade | rarely openable windows in every room, and the build-up frequently fails today's thermal requirements for residential use |
These four points must be examined before any cost estimate. If one of them fails, the conversion is not more expensive but simply not deliverable in that form.
Why benchmarks are least useful here
New build has benchmarks by building type. Renovation has benchmarks by depth of intervention. Change of use has neither, because the governing quantity is neither floor area nor intervention depth but the distance between the requirement level of the old use and that of the new one.
Two conversions of the same area in the same building can differ several-fold depending on which step change is triggered. A benchmark per square metre for a change of use is therefore at best an indication of order of magnitude and never a computational input.
What does work instead: a cost estimate proceeding from the schedule of requirements rather than from the floor area. The sequence runs: fix the new use, establish the applicable requirements, compare against the existing building, derive the schedule of measures, and only then calculate.
Feasibility precedes cost estimating
For a change of use the first question is not what it costs but whether it can be consented. Four points need settling for that in advance. All four precede any cost estimate.
| # | Point | To be settled |
|---|---|---|
| 1 | Planning law | is the new use permissible in the designated area category, and are the site coverage and floor space ratios sufficient? |
| 2 | Parking | the new use triggers a fresh obligation under municipal bylaws; on a developed site this is frequently unachievable, hence commuted payments |
| 3 | Escape routes | can the second escape route be formed within the existing building? |
| 4 | Heritage protection | where the building or ensemble is listed, the heritage authority must be involved in every case |
- Planning law. Is the new use permissible in the designated zone, and are the site coverage and floor space ratios sufficient?
- Parking. The new use triggers a new parking obligation under municipal bylaws. On a developed site it frequently cannot be met, leading to commuted payments.
- Escape routes. Can the second escape route be formed within the existing building?
- Heritage protection. Where the building or the group is protected, the heritage authority must be involved in every case.
If any of these fails, the cost estimate is moot. The effort for this preliminary check falls in cost group 700 and is the most economic expenditure in the whole scheme.
What applies to the contingency
The stated contingency described in the article on full refurbishment applies with greater force to changes of use, and with an additional distinction. It covers different risks from the contingency for a change of use.
Alongside physical uncertainty, an administrative one arises here: requirements can be specified more precisely or tightened during the consent process, particularly on fire safety and accessibility. That second uncertainty does not fall through opening up elements but only with the consent itself.
It therefore makes sense to carry both contingencies separately and to tie their release to different events. The two clear at different points in the project.
| Contingency | Covers | Released by |
|---|---|---|
| Physical contingency | structure, contamination, concealed fabric | completion of the condition survey |
| Administrative contingency | tightening of fire safety and accessibility during the process | the consent decision |
A single combined contingency conceals which part of the risk has already been cleared. It is also regularly consumed too early.
Related articles
The relationships described are orientations for Germany. They do not replace a project-specific cost estimate, nor a legal assessment of consentability in the individual case.